fix: handle corrupted session files gracefully in getChatHistoryImpl (fixes #317566)

[vs-code-engineering]

Summary

The Copilot CLI session service throws an unhandled error when sessionManager.getSession() encounters a corrupted session file on disk (e.g., invalid ephemeral field value). The error "Session file is corrupted (line 51: ephemeral: Invalid literal value, expected true)" propagates up from the @github/copilot/sdk through getChatHistoryImpl → getChatHistory → getSessionHistory → provideChatSessionContent to the VS Code extension host as an unhandled rejection. This affects users on Windows and Mac running VS Code 1.121.0.

Fixes #317566
Recommended reviewer: @meganrogge

Culprit Commit

This is a pre-existing issue — the getChatHistoryImpl method was written without error handling for the getSession() SDK call. The error surfaces when a user's local session file becomes corrupted (invalid schema values). No specific regression commit introduced this; the missing error handling has been present since the code was introduced.

Code Flow

sequenceDiagram
    participant ExtHost as extHostChatSessions
    participant Provider as copilotCLIChatSessionsContribution
    participant Service as copilotcliSessionService
    participant SDK as copilotSDK

    ExtHost->>Provider: provideChatSessionContent()
    Provider->>Service: getChatHistory()
    Service->>SDK: sessionManager.getSession()
    Note over SDK: Session file on disk has<br/>invalid ephemeral field
    SDK-->>Service: throws Error("Session file is corrupted...")
    Note over Service: No catch block exists
    Service-->>ExtHost: unhandled rejection propagates
    Note over ExtHost: Error reaches telemetry

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Affected Files

File	Role	Evidence
extensions/copilot/src/extension/chatSessions/copilotcli/node/copilotcliSessionService.ts	crash site + fix location	L1031: const session = await sessionManager.getSession({ sessionId }, false)
extensions/copilot/node_modules/@github/copilot/sdk/index.js	error origin (external SDK)	throws "Session file is corrupted" when parsing invalid session data

Repro Steps

1. Open VS Code with the Copilot extension active
2. Have a chat session file on disk that contains an invalid ephemeral field (e.g., a value other than true on line 51)
3. Trigger any action that loads chat session history (e.g., open chat panel, switch sessions)
4. The SDK throws a validation error that propagates as an unhandled error

How the Fix Works

Chosen approach (copilotcliSessionService.ts): Added a catch clause to the existing try block around sessionManager.getSession(). When the SDK throws (e.g., due to file corruption), the error is logged via logService.error() (preserving telemetry visibility) and empty history is returned — treating the corrupted session the same as a missing session. This fixes at the boundary where VS Code interacts with the SDK (the data producer boundary), rather than suppressing errors downstream. The pattern matches the existing approach in sessionReindexer.ts (line 101) where corrupt sessions are logged and skipped.

Alternatives considered: Attempting to repair/delete the corrupted session file was rejected — the SDK owns the file format and lifecycle, and VS Code should not manipulate it directly.

Recommended Owner

@meganrogge — author of the copilotcliSessionService.ts code and active contributor (commits within last 90 days). Owns Copilot terminal/chat integration areas.

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