fix: auto-migrate v1 condensing prompt and handle invalid providers on import

[hannesrudolph]

Summary

This PR introduces two improvements to configuration handling:

1. Auto-migration of old v1 default condensing prompt - Detects and clears the old v1 default condensing prompt from customSupportPrompts.CONDENSE using fingerprinting, allowing users to automatically receive the improved v2 default (which includes analysis tags, error tracking, and better task continuity).

2. Lenient settings import with invalid provider handling - The settings import system now gracefully handles invalid or removed API providers instead of failing the entire import.

Changes

• Add migrateOldDefaultCondensingPrompt() migration that runs on ContextProxy initialization
• Add isOldV1DefaultCondensePrompt() fingerprinting function to detect v1 default without exact matching
• Implement sanitizeProviderConfig() to strip invalid/removed apiProvider values from imported profiles
• Validate each provider profile individually using providerSettingsWithIdSchema.safeParse()
• Collect and return warnings for profiles that had issues (invalid providers or schema errors)
• Show showWarningMessage notification when import succeeds with warnings
• Fail gracefully only when ALL profiles are invalid, with detailed error messages
• Add default URI to import file picker (uses last export path or falls back to Downloads folder)
• Add comprehensive tests for both migrations and all import edge cases

---

Important

This PR auto-migrates old v1 condensing prompts and handles invalid providers during import, enhancing configuration handling and user experience.

• Behavior:
  • Auto-migrates old v1 default condensing prompt in ContextProxy.ts using migrateOldDefaultCondensingPrompt().
  • Handles invalid/removed API providers in importExport.ts using sanitizeProviderConfig().
  • Imports settings with warnings for invalid profiles, only failing if all profiles are invalid.
• Functions:
  • Adds isOldV1DefaultCondensePrompt() in ContextProxy.ts for prompt detection.
  • Adds importSettingsFromPath() in importExport.ts for lenient import handling.
• Tests:
  • Adds tests for prompt migration and provider handling in ContextProxy.spec.ts.
  • Adds tests for import scenarios in importExport.spec.ts.

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[roomote]

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Review status: The remaining edge cases are addressed; no further changes requested.

• Ensure currentApiConfigName is only set to an imported profile that actually survived per-profile validation (fallback if the imported current profile was skipped).
• Avoid dumping all import warnings into a single VS Code toast; prefer a short summary and log full details elsewhere (or truncate).

Previous reviews

• 9a6e386: Review #1

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}