chore: fix mypy type checking in local development

[jtroup]

what

This PR fixes mypy type checking to work consistently in local development environments and CI:

1. Added null check in stacklet/mcp/lifespan.py: Added an assertion to ensure ctx.request_context is not None before accessing its attributes (line 51)

2. Added pip to dev dependencies: mypy requires pip to auto-install type stubs, but uv venvs don't include it by default

3. Added explicit type stub packages: Added types-cachetools, types-pyyaml, and types-requests to dev dependencies for reproducible builds with pinned type versions

why

The Problem:

• CI was failing with a mypy error: Item "None" of "RequestContext[Any, Any, Request] | None" has no attribute "lifespan_context"
• The error only appeared in CI, not in local development

Root Cause:
When running mypy in a uv-managed venv without pip, mypy attempts to auto-install type stubs but silently fails. This causes mypy to continue without full type information, resulting in a false positive "Success" even when type errors exist.

The Fix:

• The code fix (assertion) satisfies mypy's strict type checking by explicitly handling the nullable type
• Adding pip ensures mypy can install additional stubs if needed (safety net)
• Adding explicit type stub packages ensures reproducible builds with known versions and prevents the silent failure scenario

This gives us the best of both worlds: reproducible builds with pinned type stubs, plus a safety net if mypy needs to install additional stubs in the future.

testing

Tested the fix by:

1. Reproduced the issue: Cleared pre-commit cache and confirmed the original error appeared in CI-like conditions
2. Verified the fix: Applied changes and ran uv run pre-commit run --all-files - all checks passed
3. Tested with and without the assertion: Confirmed that with type stubs installed, mypy correctly catches the type error when the assertion is removed

All pre-commit hooks now pass including:

• mypy type checking
• ruff linting and formatting
• pyproject-fmt formatting

docs

No user-facing documentation needed. This is an internal development tooling fix.

The inline comments in pyproject.toml explain why pip and type stubs are included in dev dependencies.

[tvansteenburgh]

Nice, thanks @jtroup