What this PR does / why we need it:

feast/infra/mcp_servers/mcp_server.py imported feast.feature_store.FeatureStore at module load, but used it only for a type annotation on add_mcp_support_to_app. Because the feast.infra.mcp_servers package __init__ eagerly imports mcp_server, importing the package pulled the entire feature_store import graph in as a side effect.

Under parallel unit-test collection (pytest -n), that heavy import intermittently raced with an in-progress feature_store import and failed collection of tests/unit/infra/feature_servers/test_mcp_server.py with KeyError: 'feast.infra.mcp_servers.mcp_server'. The tests pass in isolation, so it shows up as a flaky unit-test-python failure.

Moving the import under TYPE_CHECKING with a forward-reference annotation removes feature_store from mcp_server's module-load path, so importing mcp_server no longer triggers that graph. Public API and the annotation are unchanged.

Which issue(s) this PR fixes:

Fixes #6633

Checks

• I've made sure the tests are passing.
• My commits are signed off (git commit -s)
• My PR title follows conventional commits format

Testing Strategy

• Unit tests
• Integration tests
• Manual tests
• Testing is not required for this change

test_mcp_server.py passes (23), and feast.infra.mcp_servers still exposes add_mcp_support_to_app and McpFeatureServerConfig. ruff and mypy clean.

Misc

Release note: NONE.

As an outside contributor I can't apply labels, so a maintainer will need to add kind/bug and run ok-to-test.