feat: Support inline code completion (FIM)

[roomote-v0]

Related GitHub Issue

Closes: #12261

Description

This PR attempts to address Issue #12261 by implementing FIM (Fill-in-the-Middle) inline code completion support for Roo Code. Feedback and guidance are welcome.

Key implementation details:

• FimTokenFormatter: Maps model families (DeepSeek, CodeLlama, StarCoder, Mistral/Codestral, Qwen) to their respective FIM special tokens. Falls back to generic <|fim_prefix|>/<|fim_suffix|>/<|fim_middle|> tokens for unknown models.

• FimApiClient: Lightweight API client supporting three endpoint patterns:

  • /v1/completions (OpenAI-compatible, DeepSeek)
  • /api/generate (Ollama native FIM)
  • /v1/fim/completions (Mistral/Codestral)

• FimCompletionProvider: VS Code InlineCompletionItemProvider with debouncing (configurable, default 300ms), LRU-style caching (50 entries, 10s TTL), request cancellation via AbortController, and silent error handling.

• FimService: Orchestrator that manages the provider lifecycle -- activates/deactivates based on settings changes, passes config updates without re-registering.

• Settings added to GlobalSettings: fimEnabled, fimProvider, fimModelId, fimBaseUrl, fimDebounceMs, fimMaxTokens, fimApiKey (stored as secret).

Design decisions:

• FIM model is configured independently from the chat model so users can use a cheap/fast FIM-specialized model (e.g., DeepSeek Coder) alongside a more capable chat model
• Uses the legacy /v1/completions endpoint rather than chat completions, as FIM models work best with raw prefix/suffix tokens
• The FIM provider is a standalone service that does not interact with the chat/task system

Test Procedure

• 40 unit tests covering all four components (FimTokenFormatter, FimApiClient, FimCompletionProvider, FimService)
• Run: cd src && npx vitest run services/fim/__tests__/
• All tests pass; lint and type checks pass across the monorepo

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: New and/or updated tests have been added to cover my changes (if applicable).
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• Yes, documentation updates are required: Users will need guidance on configuring FIM settings (provider, model, API key, base URL). A settings UI section is planned as a follow-up.

Additional Notes

• This is the backend/core implementation. A settings UI (webview section for toggling FIM and configuring provider/model) would be a natural follow-up PR.
• The vscode mock was extended with InlineCompletionTriggerKind, InlineCompletionItem, and registerInlineCompletionItemProvider to support these tests.

Interactively review PR in Roo Code Cloud