[Repo Assist] feat: enhance textDocument/implementation for .fsi ↔ .fs navigation

[github-actions]

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Closes #1473

Summary

This enhances TextDocumentImplementation (textDocument/implementation) to support bi-directional navigation between .fs implementation files and .fsi signature files, using FCS-provided symbol properties.

Behaviour

Current file	Symbol has…	Result
.fsi	ImplementationLocation	Navigates to .fs implementation
.fs	SignatureLocation pointing to .fsi	Navigates to .fsi signature
Either	Neither (e.g. abstract/virtual member)	Falls back to existing dispatch-slot implementation lookup

The fallback preserves all existing textDocument/implementation behaviour for virtual/abstract member overrides.

Implementation

FCS already exposes two properties on FSharpSymbol:

• ImplementationLocation: the .fs location for a symbol declared in a .fsi file
• SignatureLocation: the .fsi location for a symbol defined in a .fs file

The change (in AdaptiveFSharpLspServer.fs, TextDocumentImplementation override):

1. Resolves the symbol at the cursor position
2. Checks whether the current file is a .fsi → if so, uses ImplementationLocation
3. Otherwise checks if SignatureLocation points to a .fsi file → navigates there
4. Falls back to the existing Commands.symbolImplementationProject path when neither applies

Test Status

Build: ✅ dotnet build src/FsAutoComplete/FsAutoComplete.fsproj -c Release — 0 warnings, 0 errors
Format: ✅ dotnet fantomas applied; re-build passes
Full test suite: deferred to CI (the change is in the LSP handler layer; no existing tests cover this code path directly)

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[github-actions]

🤖 Repo Assist here.

The 2 macOS failures are not caused by this PR's changes:

• This PR only modifies 1 file: AdaptiveFSharpLspServer.fs (the textDocument/implementation handler)
• The failing tests are all in "Go to definition tests.GoTo Tests" — a completely different code path
• All failures show the same error: Directory not empty : .../GoToTests/obj/Debug — a macOS filesystem cleanup issue where Directory.Delete(path, recursive: false) fails if the directory has pre-existing build artifacts

This is a known intermittent macOS test infrastructure issue, not related to the .fsi ↔ .fs navigation feature. All Linux and Windows runs pass cleanly.

The PR is ready for review on its merits.

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