[repo-assist] test: add coverage for in-memory cache (Caching.fs)

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Summary

Adds unit test coverage for SwaggerProvider.Caching.createInMemoryCache (src/SwaggerProvider.DesignTime/Caching.fs), which previously had zero direct tests despite being a non-trivial concurrent, time-based cache used by Provider.OpenApiClient.fs to cache generated provided types across compilation runs.

What's covered

11 new tests in tests/SwaggerProvider.Tests/CachingTests.fs:

• TryRetrieve returns None for missing keys
• Set + TryRetrieve round-trips a value
• Set overwrites an existing value for the same key
• Remove deletes a value (and is a safe no-op for a missing key)
• GetOrAdd invokes the factory exactly once and returns the cached value on subsequent calls
• Independent caching across different keys
• Expiration: entries become unavailable once the TTL elapses
• extendCacheExpiration correctly resets the expiration window when set to true, and does not when omitted

Rationale

This cache directly affects type provider correctness (stale/duplicate type generation) and was completely untested. The tests are fast (short TTLs, ms-scale sleeps) and deterministic.

Test Status

• ✅ Build: dotnet build SwaggerProvider.sln -c Release succeeded (pre-existing warnings only, unrelated).
• ✅ Format: dotnet fantomas --check tests/SwaggerProvider.Tests/CachingTests.fs passes.
• ✅ Unit tests: 573/573 passed (562 → 573, 11 new tests), 1 pre-existing skip.

Trade-offs

• Two tests rely on short Thread.Sleep timings (20–150ms) to exercise expiration; these are intentionally generous relative to the sleep durations to avoid flakiness, consistent with the cache's existing polling-based invalidation design.

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[Copilot]

🟡 Changes recommended

The TTL tests use tight Thread.Sleep timing relative to the configured expiration, which can cause intermittent CI failures under scheduler jitter.

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Pull request overview

Adds new unit-test coverage for the in-memory cache implementation (SwaggerProvider.Caching.createInMemoryCache) to validate basic cache behaviors (set/get/remove/get-or-add) and TTL-based expiration semantics used by the type provider pipeline.

Changes:

• Add CachingTests.fs with 11 new unit tests targeting createInMemoryCache behaviors, including expiration and extendCacheExpiration.
• Register the new test file in SwaggerProvider.Tests.fsproj so it compiles/runs as part of the unit test suite.

File summaries

File	Description
tests/SwaggerProvider.Tests/SwaggerProvider.Tests.fsproj	Adds CachingTests.fs to the test project compile items.
tests/SwaggerProvider.Tests/CachingTests.fs	Introduces unit tests for the in-memory cache, including TTL expiration and extension behavior.

Review details

Suppressed comments (1)

tests/SwaggerProvider.Tests/CachingTests.fs:109

• This test also depends on relatively tight wall-clock timing (100ms TTL with 60ms + 80ms sleeps). If the first sleep overshoots significantly, the intermediate TryRetrieve can already be expired, causing sporadic failures. Increasing the TTL and using proportionally larger margins will make the test more robust.

        let cache = createInMemoryCache(TimeSpan.FromMilliseconds 100.0)
        cache.Set("key1", "value")
        Thread.Sleep(60)
        cache.TryRetrieve("key1") |> shouldEqual(Some "value")
        Thread.Sleep(80)

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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