Client Auth: DPoP Sender-Constrained Tokens (SEP-1932) (#394)

* Add DPoP client conformance scenario + shared DPoP helpers (SEP-1932, #368)

Client-anchored packaging: carries the shared DPoP foundation (dpopProof /
dpopToken helpers, the createAuthServer DPoP core, and sep-1932.yaml) together
with the client scenario, so it can merge first; the server (#369) and
authorization-server (#370) scenarios are follow-ups that depend on it.

DPoP is treated as a draft/not-yet-official feature (mirroring the WIF SEP-1933
scenario): the scenario's source is `introducedIn: DRAFT_PROTOCOL_VERSION` and
it is registered in draftScenariosList (informational, not scored). It is NOT
registered in EXTENSION_IDS, since the ext-auth DPoP extension is still on a
branch rather than merged.

Client checks (the two extension MUSTs, RFC 9449 §7.1 / §4.2-4.3):
- sep-1932-client-dpop-auth-scheme — token presented with the DPoP scheme.
- sep-1932-client-fresh-proof — a fresh, well-formed DPoP proof per request.

The test MCP server judges the client via an opt-in DPoP middleware
(dpopResourceAuth) passed through createServer's authMiddleware hook; a compliant
example client (covered by the Client Draft Scenarios loop) plus deliberately-
broken bearer/replay variants drive the pass/fail acceptance tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* client/dpop: split into nonce-less baseline + nonce-required scenario

Server-provided nonces are optional in RFC 9449 (AS §8 "MAY", RS §9 "can
also choose"), and the nonce-less flow is the common case. The single
auth/dpop scenario hard-wired nonce enforcement on both the test AS and
MCP server, so a plain nonce-less proof was never accepted end-to-end and
a client that only does plain proofs was never exercised on its happy path.

Parameterize DPoPClientScenario with `requireNonce` and register it twice:
- auth/dpop        (requireNonce=false) — nonce-less baseline; emits the
  three baseline checks (token-request-proof, dpop-auth-scheme, fresh-proof).
- auth/dpop-nonce  (requireNonce=true)  — prior behavior; adds as-nonce and
  rs-nonce (five checks).

Check IDs are reused (no new IDs, no sep-1932.yaml change); the two nonce
checks are emitted only by auth/dpop-nonce, so traceability stays intact.

Add auth-test-dpop-no-nonce.ts (nonce-incapable but otherwise compliant)
asserting SUCCESS against auth/dpop — proof that a client with no nonce
handling still completes DPoP when the server does not require a nonce.
Baseline negatives stay on auth/dpop (nonce-independent); the two nonce
negatives are re-pointed to auth/dpop-nonce.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* client/dpop: round-4 nonce-surface correctness fixes

Address review findings on the auth/dpop-nonce posture:

- Record the token-request proof observation BEFORE the §8 nonce challenge,
  so a client that is challenged but does not retry is no longer mis-reported
  by token-request-proof as "never completed a token request" (its proof was
  just validated). (#2)
- Record the resource-request jti/replay on ANY valid proof (before the §9
  nonce gate), so fresh-proof is never asserted on zero evidence and a client
  that never honours the RS nonce fails ONLY rs-nonce. (#5)
- Require challengeIssued && honored for as-nonce/rs-nonce SUCCESS, closing a
  vacuous-pass path (a client that pre-sends the nonce is never challenged). (#3a)
- Gate the AS nonce observation on the authorization_code exchange, matching
  recordTokenRequestProof (a refresh exchange must not satisfy §8). (#3c)
- Collapse duplicate shared token-flow check IDs (token-request, pkce-*) that
  the §8 round-trip re-POST would otherwise emit twice. (#4)
- Example client: retry the token request only on a use_dpop_nonce error code,
  not any 400 carrying a DPoP-Nonce header (consistent with the RS path). (#6)
- Fix the DpopTokenRequestObservation docstring ("last write wins" →
  sticky-failure). (#7)

Adds an expectedSuccessSlugs option to the client test helper and tightens the
two nonce negative tests to pin the now-clean behavior (token-request-proof
SUCCESS for no-as-nonce; only rs-nonce fails for no-rs-nonce).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* client/dpop: round-5 fixes — dedupe masking, wording, replay decision

- dedupeSharedChecks was masking failures: it kept the last occurrence of a
  duplicate check ID regardless of status and ran in BOTH postures, so a
  FAILURE on a challenged POST (or an earlier attempt in a restarted flow) was
  silently dropped. Replace with an exported, unit-tested collapseDuplicateChecks
  that prefers the MOST-SEVERE occurrence, and only apply it in the nonce
  posture (the baseline keeps genuinely distinct repeated attempts). (R5 #1)
- Reword the token-request-proof description: it can be SUCCESS for a client
  that presented a valid proof but never obtained a token (challenged, no
  retry), so it no longer claims "obtaining a DPoP-bound access token". (R5 #2)
- Reword the as-nonce/rs-nonce failure messages to cover a client that DID
  retry but whose retry proof was rejected (not only "did not retry"). (R5 #3)
- Document the deliberate replay decision: recording jti on challenged requests
  means re-sending the identical proof across the challenge/retry boundary trips
  replay detection — a genuine RFC 9449 §4.2 jti-uniqueness violation. (R5 #5)

Adds src/scenarios/client/auth/dpop.test.ts unit-testing collapseDuplicateChecks
(prefers failures, keeps INFO, preserves order) — the dedupe fix is now pinned.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* client/dpop: round-6 polish — pin collapse wiring; docstring

- Add an integration assertion that the §8/§9 double-POST is collapsed to one
  token-request/pkce-* entry each in auth/dpop-nonce, so the requireNonce gating
  of collapseDuplicateChecks can't be silently deleted or inverted (previously
  mutation-survivable). runClientAgainstScenario now returns the emitted checks
  so callers can make occurrence-level assertions (backward-compatible).
- Docstring: note the severity ladder ranks SKIPPED below SUCCESS, and that
  equal-severity ties keep the last occurrence.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/scenarios/client/auth/spec-references.ts

@@ -113,5 +113,38 @@ export const SpecReferences: { [key: string]: SpecReference } = {
   SEP_1933_WIF: {
     id: 'SEP-1933-Workload-Identity-Federation',
     url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1933'
+  },
+  // DPoP (SEP-1932 / RFC 9449) — client concerns.
+  SEP_1932_DPOP: {
+    id: 'SEP-1932-DPoP',
+    url: 'https://github.com/modelcontextprotocol/modelcontextprotocol/pull/1932'
+  },
+  DPOP_EXTENSION: {
+    id: 'MCP-DPoP-Extension',
+    url: 'https://github.com/modelcontextprotocol/ext-auth/blob/pieterkas-dpop-extension/specification/draft/dpop-extension.mdx'
+  },
+  RFC_9449_PROOF_SYNTAX: {
+    id: 'RFC-9449-dpop-proof-jwt-syntax',
+    url: 'https://www.rfc-editor.org/rfc/rfc9449.html#section-4.2'
+  },
+  RFC_9449_CHECKING_PROOFS: {
+    id: 'RFC-9449-checking-dpop-proofs',
+    url: 'https://www.rfc-editor.org/rfc/rfc9449.html#section-4.3'
+  },
+  RFC_9449_AUTH_SCHEME: {
+    id: 'RFC-9449-dpop-authentication-scheme',
+    url: 'https://www.rfc-editor.org/rfc/rfc9449.html#section-7.1'
+  },
+  RFC_9449_TOKEN_REQUEST: {
+    id: 'RFC-9449-dpop-access-token-request',
+    url: 'https://www.rfc-editor.org/rfc/rfc9449.html#section-5'
+  },
+  RFC_9449_AS_NONCE: {
+    id: 'RFC-9449-authorization-server-provided-nonce',
+    url: 'https://www.rfc-editor.org/rfc/rfc9449.html#section-8'
+  },
+  RFC_9449_RS_NONCE: {
+    id: 'RFC-9449-resource-server-provided-nonce',
+    url: 'https://www.rfc-editor.org/rfc/rfc9449.html#section-9'
   }
 };