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/auth/store.ts

@@ -7,6 +7,7 @@ import type {
   OAuthClientInformation,
   OAuthTokens,
   OAuthMetadata,
+  OAuthDiscoveryState,
 } from "@modelcontextprotocol/client";
 import type {
   IdpSessionState,
@@ -15,19 +16,57 @@ import type {
 import type { OAuthPersistSnapshot } from "./oauth-persist.js";
 
 /**
- * OAuth state for a single server
+ * OAuth credentials bound to a single authorization-server `issuer` (SEP-2352).
+ *
+ * Client identifiers are unique to the AS that issued them (RFC 6749 §2.2), and
+ * tokens minted by one AS must never be presented to another. The SDK v2 stamps
+ * `issuer` on the objects it saves and passes `ctx.issuer` on every read/write;
+ * this record is how the Inspector keys that state per authorization server so a
+ * resource that migrates between ASes re-registers/re-authorizes cleanly instead
+ * of reusing mismatched credentials.
  */
-export interface ServerOAuthState {
+export interface IssuerBoundOAuthState {
   clientInformation?: OAuthClientInformation;
   /** Set when {@link clientInformation} is saved — DCR vs CIMD. */
   clientRegistrationKind?: OAuthClientRegistrationKind;
-  preregisteredClientInformation?: OAuthClientInformation;
   tokens?: OAuthTokens;
+}
+
+/**
+ * OAuth state for a single server.
+ *
+ * Issuer-bound credentials (client info + tokens) live under {@link byIssuer},
+ * keyed by AS `issuer` (SEP-2352). Per-server / per-flow state (code verifier,
+ * discovered metadata, discovery state, static client info, EMA marker, and the
+ * requested-scope seed) is issuer-independent and stays at the top level.
+ *
+ * The bare top-level {@link clientInformation} / {@link tokens} fields are the
+ * **legacy unkeyed fallback**: a pre-1625 snapshot deserializes with credentials
+ * here and no `issuer` stamp. Reads fall back to them when {@link byIssuer} has
+ * no entry; the first issuer-stamped save promotes them into {@link byIssuer} and
+ * clears the fallback (lazy migration — see `OAuthStorageBase`). New saves never
+ * write the bare fields.
+ */
+export interface ServerOAuthState {
+  /** Per-issuer credentials (SEP-2352). */
+  byIssuer?: Record<string, IssuerBoundOAuthState>;
+  /** Most-recently-saved issuer — answers ctx-less reads (per-request bearer token). */
+  activeIssuer?: string;
+  preregisteredClientInformation?: OAuthClientInformation;
   codeVerifier?: string;
   scope?: string;
   serverMetadata?: OAuthMetadata;
+  /** RFC 9728/8414 discovery cache; persisted alongside {@link codeVerifier} (SEP-2352 callback-leg binding). */
+  discoveryState?: OAuthDiscoveryState;
   /** Set when resource tokens were minted via EMA (legs 2–3). */
   enterpriseManaged?: boolean;
+
+  /** @deprecated Legacy unkeyed fallback — see {@link ServerOAuthState}. */
+  clientInformation?: OAuthClientInformation;
+  /** @deprecated Legacy unkeyed fallback — see {@link ServerOAuthState}. */
+  clientRegistrationKind?: OAuthClientRegistrationKind;
+  /** @deprecated Legacy unkeyed fallback — see {@link ServerOAuthState}. */
+  tokens?: OAuthTokens;
 }
 
 /**