feat: improve apply_code tool with isolated API context for diff generation

[roomote]

This PR addresses the subtle but important difference pointed out by @CberYellowstone in issue #6159. The original proposal emphasized creating a truly isolated, dedicated process for diff generation in the apply_code tool.

Changes

• Created isolated API handler for Stage 2: Instead of reusing the main Cline API instance, Stage 2 now creates its own isolated API handler using buildApiHandler(). This ensures complete separation from the main conversation context.

• Hardcoded optimized system prompt: The diff generation stage now uses a hardcoded, specialized system prompt focused solely on generating accurate diffs, without any conversational elements.

• Clean context without conversation history: Stage 2 receives only the essential information (original file content and new code) without any conversation history or additional context that could influence the diff generation.

• Updated tests: All tests have been updated to reflect the new implementation and are passing.

Key Differences from Previous Implementation

The previous implementation reused the main Cline API instance with a modified system prompt. This new implementation creates a completely separate API handler, ensuring true isolation as originally proposed.

Testing

• ✅ All unit tests passing
• ✅ Linting passing
• ✅ Type checking passing

Fixes #6159

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Important

Introduces apply_code tool for improved code application using a two-stage process with isolated API context, updates settings, descriptions, and tests.

• Behavior:
  • Introduces apply_code tool in applyCodeTool.ts for code changes using a two-stage process: code generation and diff generation.
  • Uses buildApiHandler() for isolated API context in diff generation.
  • Updates presentAssistantMessage.ts to handle apply_code tool execution.
• Prompts and Descriptions:
  • Adds getApplyCodeDescription() in apply-code.ts for tool description.
  • Updates index.ts to include apply_code in tool descriptions.
• Settings and Configuration:
  • Adds applyEnabled setting in provider-settings.ts to control tool availability.
  • Updates tool.ts to include apply_code in tool names.
• Testing:
  • Adds applyCodeTool.spec.ts for unit tests covering parameter validation, API workflow, error handling, and user approval.
• Misc:
  • Updates ExtensionMessage.ts and tools.ts to include apply_code in relevant interfaces and constants.

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[daniel-lxs]

I don't think this implementation makes sense, I feel like if we ever implement this it should output the whole file with the updated content, this seems to have better results than applying a diff

[bhaktatejas922]

use morphs fast apply model for edits like kilo? https://morphllm.com