[Repo Assist] fix: show default parameter values concisely in signature help

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Closes #1310

Problem

When a method has C#-style optional parameters with [(System.Runtime.InteropServices.Optional; System.Runtime.InteropServices.DefaultParameterValue(...))] attributes, the signature help parameter documentation showed the full verbose attribute syntax:

[(System.Runtime.InteropServices.Optional; System.Runtime.InteropServices.DefaultParameterValue(""))] args: string
```

## Fix

Add `TipFormatter.cleanParameterDisplay` which:
1. Detects `[<...>]` attribute blocks in the parameter display text
2. Extracts any `DefaultParameterValue(value)` argument
3. Strips the attribute block and appends the default value as `= value`

The result for the example above becomes:
```
args: string = ""

If no DefaultParameterValue is present (only [(Optional)]), the attribute block is removed cleanly.

The fix applies to TextDocumentSignatureHelp parameter documentation, which is what editors display to highlight each parameter as the user types arguments.

Changes

• src/FsAutoComplete.Core/TipFormatter.fs — add cleanParameterDisplay : TaggedText[] -> string
• src/FsAutoComplete.Core/TipFormatter.fsi — expose in the module signature
• src/FsAutoComplete/LspServers/AdaptiveFSharpLspServer.fs — use cleanParameterDisplay in TextDocumentSignatureHelp

Trade-offs

The regex \[<[^\]]*>\] handles the common case of simple attribute arguments (string/number/bool literals). Pathological cases where attribute constructor arguments contain ] (extremely rare in practice for DefaultParameterValue) would not be transformed, leaving the original verbose text as a safe fallback.

Test Status

• dotnet build src/FsAutoComplete.Core/FsAutoComplete.Core.fsproj -f net8.0 → Build succeeded (0 errors, 0 warnings)
• Full solution build hit memory limits in this CI environment (infrastructure OOM, not a code issue — consistent with other recent PRs in this repo)

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[Krzysztof-Cieslak]

/repo-assist add tests

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Commit pushed: 9c262da

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Added 7 unit tests for TipFormatter.cleanParameterDisplay to test/FsAutoComplete.Tests.Lsp/TipFormatterTests.fs:

Test case	Input	Expected output
Full verbose C# interop attributes	[(System.Runtime.InteropServices.Optional; System.Runtime.InteropServices.DefaultParameterValue(""))] args: string	args: string = ""
Short-form with string default	[(Optional; DefaultParameterValue(""))] args: string	args: string = ""
Numeric default	[(Optional; DefaultParameterValue(42))] count: int	count: int = 42
Boolean default	[(Optional; DefaultParameterValue(false))] flag: bool	flag: bool = false
Optional-only (no default)	[(Optional)] args: string	args: string
No attribute block	args: string	args: string
Multiple TaggedText segments	split across 3 tokens	flag: bool = true

All 15 TipFormatter tests (8 pre-existing + 7 new) pass. ✅

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