Improve Copilot Cloud Sessions error handling and messaging

[osortega]

This pull request enhances the error handling and messaging for the Copilot Cloud Sessions feature, specifically addressing how task states are communicated to users.

Changes Made:

• Unified the error message format to Copilot stopped: <reason>, where <reason> is derived from the task state (e.g., "cancelled", "timed out", or specific error details).
• Updated the formatTaskStoppedMessage function to reflect the new messaging format.
• Cleaned up verbose diagnostic logging in the extractTaskErrorDetail function, removing unnecessary parameters and logs.
• Restored assertions in the test for synthesizing a single turn that were accidentally removed in a previous edit.
• Updated tests to match the new message format and ensure all scenarios are covered.

Rationale:

These changes improve user experience by providing clearer and more accurate feedback regarding task states, particularly distinguishing between cancellation and failure. The logging cleanup also enhances maintainability by reducing noise in the logs. All tests have been updated and pass successfully, ensuring the reliability of the new implementation.

[Copilot]

Pull request overview

This PR improves error handling and messaging for the Copilot Cloud Sessions (Task API v2) feature. Previously, a task that terminally failed while its latest session state stayed in_progress/queued (e.g. "Failed to launch agent") would render a perpetual "Session is in progress…" spinner and cause the live streamer to poll forever. The change keys "is the task still running?" off the task-level state and surfaces a unified Copilot stopped: <reason> notice, where the reason is a concrete error detail when available, otherwise a state-derived word ("cancelled" / "timed out" / "an error occurred").

Changes:

• Adds shared isActiveTaskState / isFailedTaskState helpers as the single source of truth for task-lifecycle state, replacing the local ACTIVE_STATES set in the streamer.
• Introduces extractTaskErrorDetail and formatTaskStoppedMessage, and wires a terminal-stop notice into both the history builder and the live TaskTurnStreamer (with renderedError de-duplication).
• Gates activeResponseCallback on the task-level active state so terminally-failed tasks don't stream/poll forever; updates and expands tests.

Show a summary per file

File	Description
extensions/copilot/src/extension/chatSessions/vscode/copilotCodingAgentUtils.ts	Adds isActiveTaskState/isFailedTaskState shared helpers for task-state classification.
extensions/copilot/src/extension/chatSessions/vscode-node/copilotCloudSessionContentBuilder.ts	Adds extractTaskErrorDetail/formatTaskStoppedMessage; renders a stop notice instead of a stuck spinner in history.
extensions/copilot/src/extension/chatSessions/vscode-node/taskTurnStreamer.ts	Settles the live stream on terminal task state and emits the stop notice; tracks renderedError to avoid duplicate detail.
extensions/copilot/src/extension/chatSessions/vscode-node/copilotCloudSessionsProvider.ts	Gates the active-response streaming callback on isActiveTaskState(task.state).
extensions/copilot/src/extension/chatSessions/vscode-node/test/copilotCloudSessionsProvider.spec.ts	Adds/updates tests for the new stopped-notice behavior and helper functions.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Medium