fix: add 'invalid_target' to AuthorizationErrorCode (RFC 8707)

[siddhirajkatkar]

Fixes #2641

RFC 8707 §2 defines invalid_target as the error code for resource
indicator mismatches. Without it, AuthorizeError(error="invalid_target")
triggers a pydantic ValidationError instead of an OAuth-compliant
error response, masking the real cause with a generic server_error.

Change

Added "invalid_target" to AuthorizationErrorCode in
src/mcp/server/auth/provider.py:

AuthorizationErrorCode = Literal[
    ...
    "temporarily_unavailable",
    "invalid_target",  # RFC 8707 §2 — resource indicator mismatch
]

No other changes required — AuthorizationErrorResponse and
AuthorizeError already accept the Literal type by reference.

This also removes the # type: ignore annotations in FastMCP's
OAuthProxy that worked around this gap.