[GitHub Copilot Agent / Gemini] Tool execution fails with 400 INVALID_ARGUMENT: Function call missing thought_signature

## Observed symptom [Copilot Agent] Tool execution fails with 400 INVALID_ARGUMENT: Function call missing thought_signature Environment / Context - Extension: GitHub Copilot / Copilot Chat - Feature: Copilot Agent Mode (Tool / PowerShell integration) Issue Summary When running Copilot Agent commands that trigger tool calling (e.g., `default_api:powershell`), the request fails after 5 retries due to an invalid API request payload. The underlying Gemini API requires a `thought_signature` parameter in `functionCall` parts, which is currently missing from the generated request. ## Literal error text ``` 400 INVALID_ARGUMENT: Function call missing thought_signature ``` ## Environment GitHub issue microsoft/vscode #339255; state=open ## Trigger / context Public GitHub report in microsoft/vscode (FRESH_7D). ## Source evidence - https://github.com/microsoft/vscode/issues/339255 (2026-10-02T09:14:56Z) — evidence_class: PRIMARY_ISSUE_REPORT — GitHub issue #339255 (open) ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:40:08.726088Z - scout_id: C52c4ffd33e ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.