## Why
Interactive MCP OAuth must not trust authorization metadata that could route an
authorization code or PKCE verifier to an unrelated token endpoint.
## What changed
- Require an advertised issuer to match the origin that served authorization
metadata.
- Validate authorization and token endpoint origins before starting both
pre-registered and dynamically registered client flows.
- Allow delegated endpoint origins when the server advertises issuer-bound
authorization responses, while retaining narrow compatibility exceptions for
existing providers.
## Testing
Add coverage for rejected untrusted metadata, accepted issuer-bound delegation,
legacy provider exceptions, and both client registration paths.
GitOrigin-RevId: 43c9f94c9f6ba2d23d7e373576cf9fa26861cfc3
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