Patch upstream Copilot session typing during CI to unblock build_and_deploy

[Copilot]

build_and_deploy was failing in the VSCode compile step due to a TypeScript portability error in upstream copilotSessionWrapper.ts (onUnhandledEvent inferred type referencing internal SessionEvent). This PR adds a targeted CI patch so the checked-out upstream source compiles reliably.

• Root cause

  • The workflow builds a fresh microsoft/vscode checkout; a recent upstream change introduced:
    • readonly onUnhandledEvent = this._onUnhandledEvent.event;
  • TypeScript rejects this inferred type in this build context and requires an explicit annotation.

• Workflow change

  • Added a pre-build step in .github/workflows/build.yml:
    • Detects vscode/src/vs/platform/agentHost/node/copilot/copilotSessionWrapper.ts
    • Applies an in-place replacement only when the matching line exists
    • Leaves other versions untouched (no-op if pattern is absent)

• Patched line

  // before
  readonly onUnhandledEvent = this._onUnhandledEvent.event;
  
  // after
  readonly onUnhandledEvent: Event<SessionEvent> = this._onUnhandledEvent.event;

• Why this is scoped

  • Change is isolated to CI workflow behavior and only affects the transient upstream checkout used during build_and_deploy.