Fix duplicate event handler registration in TerminalChatService

[Copilot]

Event handlers for onDidDisposeSession and onDidChangeInstances were registered inside registerTerminalInstanceWithToolSession, creating duplicate listeners on each call. This caused memory leaks and redundant cleanup operations.

Changes:

• Move both event handlers to constructor (registered once)
• Update onDidDisposeSession handler to look up terminal instances by session ID from _terminalInstancesByToolSessionId map

// Before: registered per-call, causing duplicates
registerTerminalInstanceWithToolSession(...) {
  this._register(this._chatService.onDidDisposeSession(...));
  this._register(this._terminalService.onDidChangeInstances(...));
}

// After: registered once in constructor
constructor(...) {
  this._register(this._chatService.onDidDisposeSession(e => {
    for (const resource of e.sessionResource) {
      const sessionId = LocalChatSessionUri.parseLocalSessionId(resource);
      const instance = this._terminalInstancesByToolSessionId.get(sessionId);
      // cleanup if found
    }
  }));
  this._register(this._terminalService.onDidChangeInstances(...));
}

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