Address review feedback: IFileSystemService, log throttling, immutable prompt result

[Copilot]

Addresses three review comments from #3959 on the promptOverrideFile debug setting.

Use IFileSystemService + URI instead of Node fs

• applyPromptOverrides now accepts Uri + IFileSystemService instead of a raw string path with fs.promises.readFile
• IFileSystemService injected into ToolCallingLoop and propagated through all 4 subclasses (McpToolCallingLoop, CodebaseToolCallingLoop, DefaultToolCallingLoop, SearchSubagentToolCallingLoop)
• Tests use MockFileSystemService instead of vi.mock('fs')

Throttle warning logs for unreadable override files

• First failure per file URI logs warn; repeated failures downgrade to trace
• Successful read clears the state so subsequent errors re-surface as warn

Avoid mutating buildPromptResult

Replaced the type-cast mutation:

// Before
(buildPromptResult as { messages: Raw.ChatMessage[] }).messages = overrideResult.messages;

// After
const effectiveBuildPromptResult = { ...buildPromptResult, messages: overrideResult.messages };

All downstream consumers (token counting, _onDidBuildPrompt.fire, ResponseProcessorContext, fetch) now reference effectiveBuildPromptResult.

Tests

• Migrated 7 existing tests from vi.mock('fs') to MockFileSystemService
• Added 2 tests for log deduplication: repeated failures and re-warn after recovery

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