fix(lsp): preserve websocket clients across tab switches

[bajrangCoder]

Closes: #2680

[greptile-apps]

Greptile Summary

The PR replaces indefinite idle-client retention with a configurable grace period, preserving an LSP connection across short tab switches while eventually disposing clients that remain unused.

• Adds a cancellable 15-second idle timer to each cached LSP client.
• Cancels pending disposal when a document reattaches.
• Adds lifecycle coverage for WebSocket and transport-based runtimes.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/cm/lsp/clientManager.ts	Adds a per-client cancellable idle grace timer and retains eventual disposal for clients with no attached files.
src/cm/lsp/types.ts	Exposes an optional finite idle grace-period configuration in client-manager options.
tests/unit/lspExternalWebSocketLifecycle.test.js	Verifies reuse during the grace period and eventual disposal for WebSocket and generic transport runtimes.

Sequence Diagram

sequenceDiagram
  participant Tab
  participant Manager as LspClientManager
  participant Timer as Idle timer
  participant Client as LSP client
  Tab->>Manager: Detach last document
  Manager->>Timer: Schedule grace period
  alt Document reattaches during grace period
    Tab->>Manager: Attach document
    Manager->>Timer: Cancel
    Manager-->>Tab: Reuse existing client
  else Client remains idle
    Timer->>Manager: Grace period expires
    Manager->>Client: onClientIdle → dispose
  end

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