fix: raise for missing context keys

[xelandernt]

Summary

Make missing global-context keys raise consistently when requested.

Changes

• Check key membership before returning a value.
• Preserve stored None values and caller defaults.
• Added non-empty-context missing-key regression coverage.

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_context_resources.py	100.00% <100.00%> (ø)
that_depends/providers/context_resources.py	100.00% <100.00%> (ø)

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