Serialize LSP stop, clean up Git listeners, and miscellaneous robustness fixes

[AhmmedSamier]

Motivation

• Prevent race conditions and resource leaks when starting/stopping the language server client.
• Ensure repository state listeners are tracked and disposed to avoid memory leaks and noisy logs.
• Improve robustness of symbol handling and search-related code and add a latency benchmark for searches.

Description

• Serialize DeepLensLspClient.stop() with a new stopPromise and ensure it waits for any pending start to complete, disposes tracked disposables, and cleans up event emitters exactly once.
• Refactor stop logic with stopClientInstance() and adjust waitForStartCompletion() null checks to avoid start/stop races.
• Track and dispose repository state listeners in GitService, improve HEAD-change logging to show the previous HEAD, and fix debounce logging to reference the previous head correctly.
• Add a helper isSymbolInformationArray() and use it in reference-code-lens.ts, simplify the label typing in search-provider.ts, remove a redundant disposal registration in extension.ts, and extend the benchmark tests with a Search Request Latency case.

Testing

• Executed the extension benchmark test suite (extension.bench.test.ts) including the new Search Request Latency benchmark as an automated test; the tests completed successfully.

---

Codex Task