[Minor-1] OAuth hardening #5684

Part of #5677

Spec link: https://modelcontextprotocol.io/specification/draft/changelog
Comprised of: Minor change 7 (iss validation — SEP-2468) + Minor change 8 (application_type — SEP-837) + Minor change 9 (issuer-bound credentials — SEP-2352)

What the spec says

• Authorization servers SHOULD include the iss parameter in authorization responses per RFC 9207, and MCP clients MUST validate a present iss against the recorded issuer before redeeming the authorization code.
• MCP clients are required to specify an appropriate application_type during Dynamic Client Registration to avoid OpenID Connect redirect URI conflicts.
• Client credentials are bound to the authorization server that issued them: clients MUST key persisted credentials by the issuer identifier, MUST NOT reuse them with a different authorization server, and MUST re-register when the authorization server changes.

Gateway work

• Add iss validation to all gateway OAuth client flows, including OBO/token-exchange paths; fail closed on mismatch.
• Set correct application_type in every DCR request.
• Re-key persisted credential storage by issuer identifier, with a migration for existing stored credentials.
• Implement re-registration on authorization server change; hard-block cross-issuer credential reuse.

Acceptance criteria

• Authorization code redemption fails closed on iss mismatch.
• Credential store keyed by issuer; cross-issuer reuse impossible.
• All DCR requests include application_type.