fix: add invalid_target to AuthorizationErrorCode per RFC 8707 §2

[ZongrongLi]

Summary

Adds "invalid_target" to AuthorizationErrorCode Literal per RFC 8707 §2.

Problem

mcp/server/auth/provider.py AuthorizationErrorCode is missing invalid_target, the OAuth 2.0 error code for resource-indicator mismatches. When downstream clients (e.g. FastMCP OAuthProxy) raise AuthorizeError(error="invalid_target", ...), pydantic rejects it with ValidationError instead of returning the proper OAuth-compliant error response.

Fix

Added "invalid_target" to the Literal type in provider.py (one line).

Verification

from mcp.server.auth.provider import AuthorizationErrorCode, AuthorizeError
from mcp.server.auth.handlers.authorize import AuthorizationErrorResponse

# invalid_target is now accepted
assert "invalid_target" in AuthorizationErrorCode.__args__
AuthorizationErrorResponse(error="invalid_target", error_description="test")
AuthorizeError(error="invalid_target", error_description="test")

Closes #2641

[Kludex]

duplicated