Refactoring: Introduce overlay/caching.ts

[henrymercer]

Refactor caching logic into overlay/caching.ts in preparation for #3809.

Risk assessment

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• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

None yet.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Rollback - Change can only be disabled by rolling back the release or releasing a new version with a fix.

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• Telemetry - I rely on existing telemetry or have made changes to the telemetry.
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• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull request overview

This PR refactors overlay database caching by moving the Actions cache upload/download and cache-key logic out of src/overlay/index.ts into a dedicated src/overlay/caching.ts module, and extracts OverlayDatabaseMode into its own file.

Changes:

• Introduce src/overlay/caching.ts (and caching.test.ts) to own overlay-base database caching operations and cache key generation.
• Extract OverlayDatabaseMode into src/overlay/overlay-database-mode.ts and update imports across the codebase.
• Remove caching-related code and tests from src/overlay/index.ts / src/overlay/index.test.ts, keeping index.ts focused on overlay file/OID helpers and constants.

Show a summary per file

File	Description
src/testing-utils.ts	Updates OverlayDatabaseMode import to the new dedicated module.
src/status-report.ts	Repoints overlay-base download stats type import to overlay/caching.
src/overlay/overlay-database-mode.ts	New home for OverlayDatabaseMode enum.
src/overlay/index.ts	Removes caching implementation, leaving overlay constants and file/OID helpers.
src/overlay/index.test.ts	Removes caching-related tests now covered by overlay/caching.test.ts.
src/overlay/caching.ts	New module containing overlay-base database cache upload/download + key generation.
src/overlay/caching.test.ts	New test suite covering overlay caching behavior and cache key stability.
src/init-action.ts	Updates imports to use overlay/caching and overlay/overlay-database-mode.
src/init-action-post-helper.ts	Updates OverlayDatabaseMode import to new module.
src/init-action-post-helper.test.ts	Updates OverlayDatabaseMode import to new module.
src/database-upload.ts	Updates OverlayDatabaseMode import to new module.
src/config-utils.ts	Updates OverlayDatabaseMode import to new module; keeps overlay constants import from ./overlay.
src/config-utils.test.ts	Updates OverlayDatabaseMode import to new module; keeps overlay constants import from ./overlay.
src/codeql.ts	Updates OverlayDatabaseMode import to new module; keeps overlay file helper imports from ./overlay.
src/analyze.ts	Updates OverlayDatabaseMode import to new module.
src/analyze-action.ts	Updates overlay caching call site to import from overlay/caching.
lib/upload-sarif-action-post.js	Generated build output reflecting the refactor.
lib/upload-lib.js	Generated build output reflecting the refactor.
lib/start-proxy-action-post.js	Generated build output reflecting the refactor.
lib/resolve-environment-action.js	Generated build output reflecting the refactor.
lib/init-action.js	Generated build output reflecting the refactor.
lib/init-action-post.js	Generated build output reflecting the refactor.
lib/autobuild-action.js	Generated build output reflecting the refactor.
lib/analyze-action.js	Generated build output reflecting the refactor.
lib/analyze-action-post.js	Generated build output reflecting the refactor.

Copilot's findings

• Files reviewed: 16/28 changed files
• Comments generated: 0