MCP Python SDK adds opt-in RFC 8707 audience checking for bearer tokens, with a deprecation warning when it is left unset

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PR #3447 (merged 2026-09-04) adds `AuthSettings.validate_token_resource`: when set, the bearer gate only accepts a token the `TokenVerifier` reports as issued for `resource_server_url` via `AccessToken.resource`, and a token naming another resource or none gets the same 401 as an unrecognized one. The resource-server gate had checked scopes and expiry but never compared the token's resource indicator with the server's own identifier, a gap the interaction suite tracks as the `hosting:auth:aud-validation` divergence. Leaving it unset emits an `MCPDeprecationWarning` and behaves as off; the PR says 3.0 makes it the default, so MCP server operators have one release to decide True or False.