Construct DEFAULT_CLIENT_INFO without building the Implementation model

The default Implementation instance was validated at import, which
force-built the deferred Implementation model on every client import
(~0.7 ms). The two literals are trusted, so model_construct produces an
equal instance (same dict, same fields set) without touching the
validator; the model now builds on the first real message instead.

Author: maxisbey

src/mcp/client/session.py

@@ -63,7 +63,9 @@
     # `attrs`/`referencing` tree) in at module scope costs every client that never validates.
     from jsonschema.protocols import Validator
 
-DEFAULT_CLIENT_INFO = types.Implementation(name="mcp", version="0.1.0")
+# Trusted literal: `model_construct` skips validation so importing this module does not
+# force-build the deferred `Implementation` model; equal to the validated instance.
+DEFAULT_CLIENT_INFO = types.Implementation.model_construct(name="mcp", version="0.1.0")
 DISCOVER_TIMEOUT_SECONDS = 10.0
 _NOTIFICATION_QUEUE_SIZE: Final = 256