Always release cache resources when async shutdown fails

[Evangelink]

Summary

A failure in an asynchronous publish or materialization task causes CacheClient.ShutdownAsync to throw before MSBuildCachePluginBase.EndBuildInnerAsync calls DisposeAsync.

Impact

Cleanup is skipped on a realistic network or placement failure. The static single-plugin semaphore and cache directory lock remain held, while cache sessions, hashers, and log handles remain undisposed. In a reused MSBuild node, later builds can be permanently rejected as though another cache instance were still running.

Evidence

src/Common/Caching/CacheClient.cs drains background tasks and throws an AggregateException when any task fails. src/Common/MSBuildCachePluginBase.cs then executes shutdown and disposal sequentially without a finally block:

await _cacheClient.ShutdownAsync(cancellationToken);
await DisposeAsync();

Async publishing and materialization are enabled by default.

Suggested fix

Guarantee disposal with exception-safe shutdown, while preserving the original shutdown failure. Add coverage with a cache client whose background operation faults and assert that all locks and resources are released.

Acceptance criteria

• DisposeAsync runs even when ShutdownAsync fails.
• The shutdown exception remains observable.
• A subsequent plugin instance in the same process can acquire the single-instance lock.
• Cache/session disposal is verified by a focused test.