C# parity: textDocument/signatureHelp is F#-only — Roslyn sidecar has no handler, host answers null

The host routes textDocument/signatureHelp to the language sidecar and maps [FS-SIGHELP] results (src/signature_help.rs). Only the F# sidecar (FCS GetMethods) implements the method; C# requests hit the 'no handler' path and resolve to null, so VS Code shows no parameter hints for C# calls.

Roslyn exposes everything needed (Microsoft.CodeAnalysis.SignatureHelp / overload resolution via SemanticModel.GetSymbolInfo + IMethodSymbol.Parameters). Implement textDocument/signatureHelp in the C# sidecar returning the shared SignatureHelpResult wire shape, and tighten the user-session e2e (tests/e2e_modules/user_session_csharp.rs, the ApplyDiscount signatureHelp block) from the null-tolerant contract to hard shape assertions when this lands.

Found while hardening the Windows user-session e2e suite.

Start with the host routing and shared wire shape in src/signature_help.rs, then locate the C# sidecar's Roslyn request handling. Use Microsoft.CodeAnalysis.SignatureHelp and SemanticModel.GetSymbolInfo with IMethodSymbol.Parameters as described, and update tests/e2e_modules/user_session_csharp.rs in the ApplyDiscount signatureHelp block. Done means C# requests return a non-null shared result and the e2e test asserts its shape.