Selector contexts

[xelandernt]

Summary

Traverse and pin only the active Selector branch during injection context preparation.

Changes

• Selector registers only its key provider and pins the active candidate with an injection-local context variable.
• Injection now recursively prepares direct, string, and type-resolved provider graphs.
• Added sync and async regressions for selected contexts, unselected async branches, nested selectors, and scope-less injection.

Checklist

• Lint and format pass (ruff)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Docs updated if behavior or public API changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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Files with missing lines	Coverage Δ
tests/providers/test_selector.py	100.00% <100.00%> (ø)
tests/test_injection.py	100.00% <100.00%> (ø)
that_depends/injection.py	100.00% <100.00%> (ø)
that_depends/providers/selector.py	100.00% <100.00%> (ø)

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