fix(resolver): make resolution cache values immutable to prevent corr…

[smoparth]

fix(resolver): make resolution cache values immutable to prevent corruption

Store session-level cache values as tuples instead of lists so that callers cannot accidentally mutate shared cache state. Return defensive list copies from resolve() and resolve_versions() to keep the public API unchanged.

Add tests verifying that cached values are immutable and that repeated resolve() calls return independent lists.

Closes: #1021

Pull Request Description

What

Why

• PR follows CONTRIBUTING.md guidelines

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 478701b and ddd1ea1.

📒 Files selected for processing (3)

• src/fromager/bootstrap_requirement_resolver.py
• src/fromager/bootstrapper.py
• tests/test_bootstrap_requirement_resolver.py

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📝 Walkthrough

Walkthrough

The change converts the internal cache in BootstrapRequirementResolver from storing mutable lists to immutable tuples, preventing accidental corruption when callers modify returned values. The cache accessor and field type annotations are updated accordingly. When return_all_versions=True, callers convert the cached tuple back to a list to maintain existing return type contracts. Three new test cases validate cache immutability, isolation between calls, and proper caching behavior.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: making resolution cache values immutable to prevent corruption.
Description check	✅ Passed	The description is directly related to the changeset, explaining what was changed (cache values as tuples), why (prevent mutation of shared state), and how (defensive copies in public API).
Linked Issues check	✅ Passed	The PR fully addresses issue #1021 by converting mutable list cache values to immutable tuples, preventing accidental corruption of shared cache state at the type level.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to the cache immutability objective: tuple storage, defensive list returns, and validation tests with no extraneous modifications.

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[rd4398]

LGTM