Resolution cache values should be immutable to prevent accidental corruption

[LalatenduMohanty]

BootstrapRequirementResolver._resolved_requirements (introduced in #975) uses mutable list values. Since get_cached_resolution returns the cached list by reference, any caller that appends, removes, or reorders items would silently corrupt the shared cache, affecting all subsequent lookups for that requirement.

Making the values immutable prevents this class of bug at the type level.

[LalatenduMohanty]

Proposed fix: change cache value type from list[tuple[str, Version]] to tuple[tuple[str, Version], ...] in:

• _resolved_requirements type annotation
• get_cached_resolution return type
• cache_resolution parameter type
• _resolve return type
• Bootstrapper call site that constructs the cached value

Internal types only — no public API changes.