feat: Add "Fix all: X" code action for codefixes appearing multiple times in a file

[Copilot]

The TypeSpec language server only offered per-instance quick fixes. ESLint-style "Fix all X problems" was missing, so users had to manually apply the same fix repeatedly for e.g. multiple unused using statements.

Changes

serverlib.ts

• currentDiagnosticIndex now stores { diagnostic, fileUri } instead of just Diagnostic, enabling file-scoped lookup without resolving AST nodes
• getCodeActions: after building individual fixes, counts how many times each codefix ID appears across all diagnostics in the same file; adds a "Fix all: <label>" action (kind QuickFix) when count > 1, carrying { fixAllInFile: { fixId, fileUri } } in data
• resolveCodeAction: handles fixAllInFile data by collecting and merging edits from every matching codefix in the target file into one workspace edit

Example

Given two unused using statements, the lightbulb menu now shows:

Remove unused code          ← fixes current diagnostic only
Fix all: Remove unused code ← removes all unused usings in the file

test/server/code-actions.test.ts — new tests covering:

• "Fix all" action appears when ≥2 instances of the same codefix exist in the file
• "Fix all" action is absent when only one instance exists
• Resolved "Fix all" action produces edits covering all instances

• Fixes Codefixes should suggest a fix all for a given file #3042

[azure-pipelines]

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[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/compiler@11468

commit: 445e442