Clean up the FeedbackClassifierService + CustomizedCodeUpdateTool

[Copilot]

Summary

Implements changes from the unresolved review conversations on PR #13642 plus a new bug fix.

Changes

Item 1 — FeedbackClassificationTemplate: language is now required

• private readonly string? _language → private readonly string _language
• Constructor parameter string? language → string language
• _language ?? "N/A" → _language in the prompt (no null-coalescing needed)
• FeedbackClassifierService.BatchClassifyAsync passes language ?? string.Empty to satisfy the non-null requirement

Item 5 — Remove plainTextFeedbackFile parameter

• CustomizedCodeUpdateTool.GetFeedbackItems accepted a plainTextFeedbackFile path that read from disk. That parameter and its file-reading code are removed; only the plain text string is supported.

Item 6 — Move feedback gathering into FeedbackClassifierService

• Added IAPIViewFeedbackService injection to FeedbackClassifierService
• Added private GatherFeedbackItemsAsync(string? apiViewUrl, string? plainTextFeedback, ct) preprocess method (the gathering logic moved from the tool)
• The gathering step is folded directly into ClassifyItemsAsync via two new optional parameters apiViewUrl and plainTextFeedback — no separate method needed. When the passed items list is empty and either source param is set, items are gathered first (mutating the caller's list) before classification proceeds.
• CustomizedCodeUpdateTool.RunUpdateAsync now calls _classifierService.ClassifyItemsAsync for the first pass with an empty list and the source params (passing language from the resolved languageService), and removes the old GetFeedbackItems method entirely
• Second-pass ClassifyItemsAsync call also now passes language

New — Detect already-applied TSP customizations

Fixes a bug where requesting a change that was already applied (e.g. running the same customization twice) returned TSP_APPLICABLE instead of SUCCESS.

Updated FeedbackClassificationTemplate prompt in three places:

1. Task instructions: Added an explicit instruction to use grep_search/read_file tools to check TypeSpec files for the requested decorator before classifying as TSP_APPLICABLE
2. Decision logic: Added a sub-step under "actionable items" to check files first; if already present → SUCCESS
3. Non-actionable list: Added "Already applied in TypeSpec" as a SUCCESS condition

Tests

• FeedbackClassifierServiceTests: added Mock<IAPIViewFeedbackService> field; passed to CreateMockedService() and CreateRealService() constructors
• CustomizedCodeUpdateToolTests: unified single ClassifyItemsAsync mock handles both passes (items.Count == 0 for first pass, items.Count > 0 for second pass); updated all configureClassifier overrides across tests accordingly

Security Summary

No security vulnerabilities introduced or discovered. CodeQL skipped (database too large).