fix: prevent unnecessary diagnostic refreshes on irrelevant watch events (#4589)

[ibesuperv]

Fixes #4589

This PR optimizes Session.DidChangeWatchedFiles to only trigger the expensive workspace/diagnostic/refresh RPC round-trip when the watch events contain files that could actually affect the TypeScript program.

Changes

• Gated Refresh: ScheduleDiagnosticsRefresh() is now only invoked if the batch contains files with relevant extensions (using the canonical extension list from isRelevantFileName), or directories (files with no extension).
• Invariant Preservation: Cache invalidation functions (cancelWarmAutoImportCache, scheduleIdleCacheClean) continue to execute unconditionally on all watch events, ensuring that package installs (which generate noise like .svg and .md) still correctly invalidate stale cache state.
• Testing: Added a comprehensive test suite in TestSession/DidChangeWatchedFiles/skips_irrelevant_extensions to verify that diagnostic refreshes correctly trigger (or skip) for .ts, tsconfig.json, directories, and irrelevant files.

Why this matters

Currently, when a user installs a package (e.g., via npm install), the LSP receives hundreds of watch events for irrelevant files like .svg, .png, and .md. Previously, this unconditionally triggered ScheduleDiagnosticsRefresh(), causing a CPU and RPC spike on the VS Code client. This patch filters the noise at the transport layer, eliminating the spurious refreshes while maintaining correctness.

[Copilot]

Pull request overview

This PR optimizes the language server’s Session.DidChangeWatchedFiles path by gating workspace/diagnostic/refresh so it only runs when a watch batch includes changes that could affect the TypeScript program, reducing client RPC/CPU spikes during noisy filesystem activity (e.g. npm install).

Changes:

• Added relevance detection in DidChangeWatchedFiles to conditionally schedule ScheduleDiagnosticsRefresh().
• Preserved unconditional cache invalidation behavior on all watch events.
• Added a Go regression test sub-suite to verify refresh behavior for relevant vs irrelevant watch events.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
internal/project/session.go	Adds relevance gating for ScheduleDiagnosticsRefresh() based on watched file URI extensions / extensionless paths.
internal/project/session_test.go	Adds a new DidChangeWatchedFiles test sub-suite covering refresh-triggering behavior across several event scenarios.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.