feat: Allow cancellation of in-progress Gemini requests and pre-execution checks

- Implements cancellation for Gemini requests while they are actively being processed by the model.
- Extends cancellation support to the  logic within tools. This allows users to cancel operations during the phase where the system is determining if a tool execution requires user confirmation, which can include potentially long-running pre-flight checks or LLM-based corrections.
- Underlying LLM calls for edit corrections (within  and ) and next speaker checks can now also be cancelled.
- Previously, cancellation of the main request was not possible until text started streaming, and pre-execution checks were not cancellable.
- This change leverages the updated SDK's ability to accept an abort token and threads s throughout the request, tool execution, and pre-execution check lifecycle.

Fixes https://github.com/google-gemini/gemini-cli/issues/531

packages/server/src/core/geminiChat.ts

@@ -155,7 +155,7 @@ export class GeminiChat {
     const responsePromise = this.modelsModule.generateContent({
       model: this.model,
       contents: this.getHistory(true).concat(userContent),
-      config: params.config ?? this.config,
+      config: { ...this.config, ...params.config },
     });
     this.sendPromise = (async () => {
       const response = await responsePromise;
@@ -219,7 +219,7 @@ export class GeminiChat {
     const streamResponse = this.modelsModule.generateContentStream({
       model: this.model,
       contents: this.getHistory(true).concat(userContent),
-      config: params.config ?? this.config,
+      config: { ...this.config, ...params.config },
     });
     // Resolve the internal tracking of send completion promise - `sendPromise`
     // for both success and failure response. The actual failure is still