#576 Resolve `FastMCP` settings before StackOne test fixtures

Summary

Resolve FastMCP's Settings model before shared StackOne fixtures construct local servers. With FastMCP 3.4.5 and pydantic-settings 2.15.0, the unresolved lifespan annotation becomes an error under the suite warnings policy.

This is test-fixture-only compatibility work. Runtime code, dependency constraints, and the lockfile are unchanged.

Linked Issue

Fixes #571

Checklist

• Linked issue exists and is referenced above
• Tests updated for the compatibility behavior
• make lint && make typecheck && make test passes locally
• make testcov passes locally with 100% coverage
• pyproject.toml and uv.lock are unchanged
• Docstrings use single backticks (not RST double backticks)

AI assistance

Prepared with AI assistance. The reproduction, diff, and validation reported above were verified in the checked-out repository.