[Repo Assist] fix: report all curried parameters in function application signature help

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Closes #744

Problem

When typing f 1 for a curried function let f a b = (), textDocument/signatureHelp returned:

{
  "signatures": [{ "label": "val f : a:'a -> b:'b -> unit", "parameters": [{ "label": "a" }] }],
  "activeParameter": 1
}

ActiveParameter is 1 but parameters has only one entry. Editors fall back to highlighting nothing (or the last known parameter) because index 1 is out of bounds for a single-element array.

Root cause

GetMethods (the FCS API used in getSignatureHelpForFunctionApplication) only exposes the first curried parameter group. For let f a b = () it returns one MethodGroupItem whose Parameters array contains only a. The code already computed argumentIndex correctly, it just didn't have enough parameter entries to back it up.

Fix

1. Add FunctionApplicationParameters: (string * string)[] option to SignatureHelpInfo — one (name, typeDisplay) pair per curried group, derived directly from mfv.CurriedParameterGroups (already in scope) using symbolUse.DisplayContext for type formatting.

2. In AdaptiveFSharpLspServer.TextDocumentSignatureHelp, use FunctionApplicationParameters when present (function application calls) instead of MethodGroupItem.Parameters.

The method-call path (getSignatureHelpForMethod) is unchanged — it sets the new field to None.

Test status

• Updated the existing issue Function application signature help: only first parameter is ever highlighted #744 test to also assert Signatures[0].Parameters.Length = 2 (previously the test only checked ActiveParameter).
• All 116 signature help tests pass (dotnet test -f net8.0 ./test/FsAutoComplete.Tests.Lsp/FsAutoComplete.Tests.Lsp.fsproj --filter "signature"); 2 skipped by design (ptestCaseAsync for issue 745).
• Build: dotnet build — succeeded, 0 warnings, 0 errors.

Trade-offs

• FunctionApplicationParameters adds a small allocation per function-application signature-help request, but only when triggered. Impact is negligible.
• Tupled parameter groups (e.g. let f (a, b) c = ()) produce a single entry per group, formatted as "(a, b)" / "(T1 * T2)" — consistent with how the signature label already presents tuples.

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