refactor(bootstrapper): apply review follow-ups to _cache module

[dhellmann]

Add docstring to Bootstrapper class describing it as an iterative DFS engine to be used as a context manager.

Rename private _cache functions that cross module boundaries to drop the leading underscore, reflecting their role as internal-but-public APIs within the bootstrapper package:

• _find_cached_wheel → find_cached_wheel
• _bg_prepare_prebuilt → bg_prepare_prebuilt

Update all call sites in _bootstrapper.py, _prepare_source.py, _resolve.py, and tests.

[coderabbitai]

📝 Walkthrough

Walkthrough

The cache helpers _find_cached_wheel and _bg_prepare_prebuilt were renamed to find_cached_wheel and bg_prepare_prebuilt. Bootstrapper, source preparation, and resolution code now use the renamed helpers. Existing tests and patch targets were updated accordingly. The Bootstrapper class also gained a docstring describing its iterative DFS behavior and context-manager flow.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title matches the main changes: bootstrapper follow-ups and _cache API renames.
Description check	✅ Passed	The description accurately summarizes the docstring addition and cross-module cache helper renames.

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