Fix invalid URI for untitled references/definitions

[Copilot]

This PR addresses the issue where find references and go-to-definition operations on untitled files return incorrect URIs that cannot be linked back to the original location.

Root Cause Analysis

The issue was in the path handling logic in internal/tspath/path.go. When VS Code sends requests with untitled URIs like untitled:Untitled-2, the following sequence occurs:

1. ✅ DocumentURIToFileName correctly converts untitled:Untitled-2 to ^/untitled/ts-nul-authority/Untitled-2
2. ❌ GetEncodedRootLength did not recognize ^/ as a rooted path, returning 0
3. ❌ ToPath treated the untitled filename as relative and resolved it against current directory
4. ❌ This corrupted ^/untitled/ts-nul-authority/Untitled-2 into /home/user/project/^/untitled/ts-nul-authority/Untitled-2
5. ❌ Later when FileNameToDocumentURI was called, it couldn't recognize the corrupted path as untitled and returned a regular file URI

Solution

Modified GetEncodedRootLength to recognize paths starting with ^/ as rooted paths (similar to how / and c: are handled). This prevents untitled filenames from being resolved against the current directory.

Implementation

• Added check for ^/ prefix in GetEncodedRootLength function
• Returns root length of 2 for untitled paths (length of ^/)
• Added comprehensive tests for edge cases and round-trip conversion
• Added integration test simulating the complete LSP flow
• Verified existing functionality is preserved

Before Fix

Input: untitled:Untitled-2
File path after ToPath: /home/user/project/^/untitled/ts-nul-authority/Untitled-2  
Output URI: file:///home/user/project/%5E/untitled/ts-nul-authority/Untitled-2

After Fix

Input: untitled:Untitled-2
File path after ToPath: ^/untitled/ts-nul-authority/Untitled-2
Output URI: untitled:Untitled-2

The integration test confirms the complete flow works correctly, simulating the exact scenario from the original issue where references and definitions on untitled files now return proper untitled: URIs instead of broken file:// URIs.

Fixes #1343.

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[DanielRosenwasser]

This is uncommon, so move it right before the URL checks.

[Copilot]

Moved the untitled path check to before the URL checks as requested. This improves performance by checking common cases (POSIX/DOS paths) first before the uncommon untitled paths. Fixed in 0fd6461.

[DanielRosenwasser]

There's a check for && !strings.HasPrefix(opts.FileName, "^/") somehwere in the code that no longer applies

[Copilot]

There's a check for && !strings.HasPrefix(opts.FileName, "^/") somehwere in the code that no longer applies

Removed the redundant check as requested. Since GetEncodedRootLength now returns 2 for ^/ paths instead of 0, the && !strings.HasPrefix(opts.FileName, "^/") condition is no longer needed. Fixed in 7d3c7e7.