Resolve #2276: cover cache-manager edge regressions

[ayden94]

Summary

Add targeted cache-manager regression coverage for documented edge contracts around Redis reset ownership, fallback eviction, non-cacheable GET results, listener cleanup, and custom stores.

Linked context: Closes #2276

Changes

• Cover empty keyPrefix Redis reset ownership across recreated store instances sharing Redis.
• Cover fallback timer deferred eviction failure containment.
• Cover undefined GET results as non-cacheable.
• Ensure unhandledRejection listener cleanup uses try/finally.
• Cover custom store get/set/reset and interceptor deletion operations.

Testing

• pnpm exec biome check packages/cache-manager/src/stores/redis-store.test.ts packages/cache-manager/src/interceptor.test.ts packages/cache-manager/src/module.test.ts
• pnpm --filter @fluojs/cache-manager... build
• pnpm --filter @fluojs/cache-manager typecheck
• pnpm --filter @fluojs/cache-manager test

Release impact

• This PR has consumer-visible release impact and includes a changeset.
• This PR has no consumer-visible release impact.

Test-only regression coverage for existing documented behavior; no .changeset/*.md file is required.

Public export documentation

No public exports changed.

• Changed public exports include a source-level summary.
• Changed exported functions document matching @param / @returns tags where applicable.
• Source @example blocks and README scenario examples still play complementary roles.

Behavioral contract

This PR preserves existing documented cache-manager behavior by adding regression tests only.

• No documented behavioral contracts were removed without migration notes.
• New behavioral contracts are documented in the affected package README.
• Intentional limitations are explicitly stated (not silently removed).
• Runtime invariants are covered by regression tests.

Platform consistency governance (SSOT)

No platform contract or governance docs changed.

• SSOT English/Korean mirror structure remains synchronized for changed governance docs.
• If platform contract docs changed, companion updates include discoverability/docs index, tooling or CI enforcement, and regression-test evidence.
• Any package README alignment/conformance claims are backed by createPlatformConformanceHarness(...) tests.