AWS Cognito access tokens do not include an `aud` claim per RFC 7519 —
they use `client_id` instead. This causes `_has_mcp_audience` to reject
all Cognito-issued tokens with "Missing MCP audience. Got []".
When `aud` is empty, fall back to the `client_id` JWT claim for audience
validation. The MCP server's own client_id will be present there since
the AS proxy exchanges the authorization code using its credentials.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>