fix(mcp): decouple AnyUrl import from mcp dependency

AnyUrl was imported inside the same try block as mcp.client.auth, so
when the mcp package was not installed, AnyUrl was undefined and
_build_client_metadata raised NameError at runtime.

Moved the AnyUrl import to its own try/except block so it's available
whenever pydantic is installed (which is a core dependency), regardless
of whether the mcp SDK is present.

Also added pytest.importorskip('mcp') to the three
test_build_client_metadata tests that exercise _build_client_metadata,
since that function depends on OAuthClientMetadata from the mcp package.

tools/mcp_oauth.py

@@ -53,7 +53,7 @@ logger = logging.getLogger(__name__)
 # Lazy imports -- MCP SDK with OAuth support is optional
 # ---------------------------------------------------------------------------
 
-_OAUTH_AVAILABLE = False
+_OAUTH_AVAILABLE=False
 try:
     from mcp.client.auth import OAuthClientProvider
     from mcp.shared.auth import (
@@ -61,12 +61,16 @@ try:
         OAuthClientMetadata,
         OAuthToken,
     )
-    from pydantic import AnyUrl
 
-    _OAUTH_AVAILABLE = True
+    _OAUTH_AVAILABLE=True
 except ImportError:
     logger.debug("MCP OAuth types not available -- OAuth MCP auth disabled")
 
+try:
+    from pydantic import AnyUrl
+except ImportError:
+    AnyUrl = None  # type: ignore[assignment, misc]
+
 
 # ---------------------------------------------------------------------------
 # Exceptions