Add FileSystem abstraction for testing

[mbg]

Mostly a few cherry-picked commits from #4221 that are not directly related to that PR and more broadly useful. The main change is the addition of the FileSystem type and a corresponding "FS" feature for ActionState that allows us to test functions which rely on the fs module more easily.

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🟢 Approval recommended

The reviewable changes consistently introduce the abstractions without identified correctness issues.

0 open findings

What changed in this PR

Adds injectable filesystem and environment abstractions to improve testability.

Changes:

• Adds FileSystem, FS action state, and OS release metadata.
• Routes CodeQL environment access through Env.
• Narrows read-only environment parameters.

File	Description
src/​fs.ts	Defines the filesystem abstraction.
src/​action-common.ts	Provides filesystem and OS metadata to actions.
src/​testing-utils.ts	Adds filesystem and OS defaults to test state.
src/​init-action.ts	Enables filesystem state for initialization.
src/​codeql.ts	Injects environment access into CodeQL operations.
src/​actions-util.ts	Accepts read-only environments for temporary paths.
src/​cli/​output-cache.ts	Accepts read-only environments for cache paths.
lib/​entry-points.js	Generated bundle; excluded from review by policy.

Files excluded by content exclusion policy (1)

• lib/entry-points.js

🧠 Review effort: Balanced

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[henrymercer]

This change lets us run tests that stub fs in parallel, at the expense of requiring a wrapper object to be threaded through the program and used instead of direct APIs. However for most tests, it's convenient to use fs APIs directly with a temporary directory, and these tests can still be run in parallel with distinct temporary directories. Therefore I don't see the benefit of this outweighing the ongoing complexity of maintaining the wrapper.