fix: fix listener leaks and redundant file read in autocomplete

[RomneyDa]

Summary

Three focused fixes for autocomplete performance — the highest impact-per-line changes from the analysis of stale PR #8364 and related issues.

• statusBar.ts: onDidChangeConfiguration was registered on every setupStatusBar() call. Since setupStatusBar is called on every config change (including autocomplete toggle), listeners accumulated exponentially — each one re-triggering setupStatusBar, which added another listener. Guard with a configListenerRegistered flag.

• NextEditWindowManager.setupListeners(): 4 VS Code event listeners were created without being added to this.disposables. The dispose() method iterates this.disposables to clean up, but these listeners weren't in it — so they survived across clearInstance()/re-creation cycles when toggling Next Edit.

• constructPrefixSuffix: Was calling ide.readFile() internally, but its only caller (HelperVars.init()) already has the file contents loaded 3 lines above. Changed the signature to accept fileContents: string directly, eliminating one redundant file read per completion request.

Related issues

Fixes #3616
Refs #5055
Refs #8364

Test plan

• Autocomplete still works end-to-end
• Toggle tab autocomplete on/off 20+ times — CPU should stay stable
• Toggle Next Edit on/off repeatedly — no performance degradation
• Existing vitest suite passes

---

Summary by cubic

Prevents listener leaks when toggling autocomplete and Next Edit, and removes a redundant file read per completion to improve stability and CPU. Fixes #3616 (refs #5055, #8364).

• Bug Fixes
  • Status bar: register onDidChangeConfiguration once with a guard to stop exponential listener growth.
  • Next Edit: add four VS Code event listeners to this.disposables so dispose() cleans them up between toggle cycles.
  • Autocomplete: make constructInitialPrefixSuffix accept preloaded fileContents (updated HelperVars) to remove an extra readFile per completion.

^{Written for commit 4794082. Summary will update on new commits.}

[continue]

Docs Review

No documentation updates needed for this PR.

Reason: These changes are internal implementation fixes (listener leak prevention and redundant file read elimination) that don't affect user-facing behavior, configuration options, or documented APIs. The autocomplete feature works exactly the same from a user's perspective — these are purely stability and performance improvements in the internal codebase.

[cubic-dev-ai]

No issues found across 4 files