fix: guard against invalid line in provideAltTextQuickFix (fixes #318227)

[vs-code-engineering]

🔧 Error Fix

Summary

The provideAltTextQuickFix method in the Copilot Chat extension's code action provider calls document.lineAt(range.start.line) without validating that the line number is within the document's bounds. When the document is modified between when VS Code computes the code action range and when the provider executes, range.start.line can exceed document.lineCount, causing TextDocument.lineAt() to throw "Illegal value for line".

This error has been present across many extension versions (0.38.2 through 0.49.0) with ~10k+ total hits affecting ~4000+ users, indicating a persistent race condition in the code action lifecycle.

Fixes #318227
Recommended reviewer: @AidanImmer

Culprit Commit

Not identified — this is a long-standing bug present across all tracked extension versions (since at least 0.38.2). The provideAltTextQuickFix method has always lacked bounds validation on the range parameter.

Code Flow

sequenceDiagram
    participant VSCode as VS Code Editor
    participant QFP as QuickFixesProvider
    participant Doc as TextDocument

    VSCode->>QFP: provideCodeActions(doc, range, ...)
    Note over VSCode: Document modified after range computed
    QFP->>QFP: provideAltTextQuickFix(doc, range)
    QFP->>Doc: lineAt(range.start.line)
    Note over Doc: line >= lineCount
    Doc-->>QFP: throws "Illegal value for line"

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Affected Files

File	Role
extensions/copilot/src/extension/inlineChat/vscode-node/inlineChatCodeActions.ts	Crash site and fix location — provideAltTextQuickFix calls lineAt() without bounds check

Repro Steps

1. Open a markdown file with image references in VS Code with Copilot Chat enabled
2. Rapidly edit the document (adding/removing lines) while the code action provider is triggered
3. The race condition between document modification and code action evaluation causes an out-of-bounds line number to be passed to lineAt()

How the Fix Works

Chosen approach (extensions/copilot/src/extension/inlineChat/vscode-node/inlineChatCodeActions.ts):

Added a guard clause at the entry of provideAltTextQuickFix that validates range.start.line is within [0, document.lineCount) before calling document.lineAt(). If the line is out of bounds, the method returns undefined early (no alt-text quick fix offered for that invocation). This follows the principle of adding guard clauses upstream of the crash site to prevent the error path from being reached, rather than wrapping in try/catch which would hide the error from telemetry.

The range is produced by VS Code's code action infrastructure and can become stale due to concurrent document edits — this is a known pattern where the consumer must validate framework-provided ranges.

Recommended Owner

@AidanImmer — owner of the Copilot Chat extension inline chat code actions based on the file path in the copilot extension area.

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