OAuth: re-key store to (server, issuer) + SEP-837/2350/2352 auth UX (#1625)

Re-key the persisted OAuth store to (server, issuer) and add the
SEP-837/2350/2352 authorization-hardening UX. The SDK v2 auth() drives the
SEP-2352 flow off the provider's ctx.issuer; the Inspector's job is to honor it
and round-trip the issuer stamp.

SEP-2352 — issuer-bound credentials
- ServerOAuthState nests issuer-bound credentials under
  byIssuer[issuer] = { clientInformation, clientRegistrationKind, tokens }, with
  an activeIssuer pointer answering the transport's ctx-less per-request bearer
  read.
- Fix a latent bug: OAuthStorageBase re-parsed tokens/client-info through
  OAuthTokensSchema/OAuthClientInformationSchema, which strip the non-RFC
  `issuer` field and silently defeat the SDK's discardIfIssuerMismatch. The
  store re-attaches the stamp from the byIssuer key.
- BaseOAuthClientProvider threads ctx.issuer and implements
  discoveryState()/saveDiscoveryState() (activates the SDK's callback-leg
  AuthorizationServerMismatchError binding check) and invalidateCredentials().
- Lazy, migration-free upgrade: a legacy pre-1625 blob's top-level
  clientInformation/tokens are returned as an unkeyed fallback until the first
  issuer-stamped save promotes them into byIssuer.

SEP-837 — application_type
- DCR declares application_type: "native" (localhost Inspector).
- ClientSettingsForm renders RegistrationRejectedError detail and reflects DCR's
  deprecated-in-favor-of-CIMD (SEP-991) status.

SEP-2350 — step-up scope union
- Per-server onInsufficientScope (reauthorize | throw) setting wired
  ServerSettingsForm -> StoredMCPServer.oauth -> StreamableHTTPClientTransport.
- StepUpAuthModal visualizes the prior-union-challenged scope set, tagging each
  scope `new` vs `already granted`.

mcpAuth simplification
- Delete the hand-rolled authorizeWithoutRefresh (mishandled issuer stamping);
  forward forceReauthorization/skipIssuerMetadataValidation to native SDK auth()
  so both the initial and step-up paths get SEP-2352 stamping.

Vocabulary + docs
- OAuthStep gains cimd_fetch / issuer_comparison / scope_step_up (recorded on a
  step-up redirect); the Network tab labels auth requests by OAuth phase.
- v2_auth_hardening.md gains an as-built status section.

Deferred to the era-aware card: the SdkError(EraNegotiationFailed) ->
UnauthorizedError unwrap and finishAuth(URLSearchParams) form (only fire under
non-legacy negotiation).

Closes #1625

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/mcp/remote/node/server.ts

@@ -1415,6 +1415,17 @@ export function createRemoteApp(
         error: "settings.enterpriseManaged must be a boolean",
       };
     }
+    if (
+      obj.oauthOnInsufficientScope !== undefined &&
+      obj.oauthOnInsufficientScope !== "reauthorize" &&
+      obj.oauthOnInsufficientScope !== "throw"
+    ) {
+      return {
+        ok: false,
+        error:
+          "settings.oauthOnInsufficientScope must be 'reauthorize' or 'throw'",
+      };
+    }
     // roots is optional on the wire (older clients won't send it); when
     // present it must be an array of `{ uri, name? }`. Reuses the same
     // module-scope `isRootArray` guard the read path applies in
@@ -1477,6 +1488,12 @@ export function createRemoteApp(
     if (obj.enterpriseManaged === true) {
       value.enterpriseManaged = true;
     }
+    if (
+      obj.oauthOnInsufficientScope === "reauthorize" ||
+      obj.oauthOnInsufficientScope === "throw"
+    ) {
+      value.oauthOnInsufficientScope = obj.oauthOnInsufficientScope;
+    }
     // Empty cwd coerces to absent on the value (matching the read side); the
     // write-through distinguishes "sent cwd: '' " (clear) from "cwd omitted"
     // (preserve) via `cwdProvided` below rather than the value alone.