Fixes silent error handling in Graph commit message generation

[sergeibbb]

Summary

• Shows an error notification when Graph commit details "Generate Commit Message" fails with an untyped provider error (e.g. Copilot credit limit)
• Adds Provider Error and Provider Unavailable AI simulator modes for reproducing untyped provider failures in debug builds

Closes gitkraken/vscode-gitlens-private#89

Approach

Implemented (Option A — targeted fix): Added showGenericErrorMessage() to the Graph's generateCommitMessage catch block in graphWebview.ts, matching the existing SCM command behavior in generateCommitMessage.ts.

Alternative considered (Option B — systemic fix): Add a fallback notification in sendRequest() at aiProviderService.ts:1632-1644 for all unclassified errors before rethrowing, so no caller has to handle it individually. This would cover all current and future AI surfaces (Graph, SCM, Composer, Review, etc.) but is a broader change in a critical path.

Test plan

• Enable subscription simulator (Pro) and AI simulator (Provider Error mode)
• Open Graph, select WIP, click sparkle "Generate Commit Message"
• Verify error notification is now shown (previously silent)
• Verify SCM panel "Generate Commit Message" still shows error notification
• Verify typed AIError modes (Error, Quota) still show their specific notifications from both surfaces
• Verify Provider Unavailable mode is silent on both surfaces (expected — no error to report)

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