feat(completion): add function-name completion

[Seddryck]

Summary

• advertise and handle standard LSP function-name completion requests
• detect valid completion positions through Expressif.Syntax parser probes and exclude literal contexts
• load authoritative names, aliases, descriptions, categories, and signatures once through Expressif.Functions.Introspection
• replace the complete typed function token with an explicit LSP text edit
• preserve Expressif XML documentation in build and publish outputs for descriptive introspection

This branch is based on feat/end-to-end-syntax-diagnostics from #15 / PR #18 as requested.

Validation

• dotnet build Expressif.LanguageServer.sln --configuration Release --no-restore --nologo -m:1 /nodeReuse:false (succeeded for .NET 8, 9, and 10 with no warnings)
• dotnet test src/Expressif.LanguageServer.Core.Tests/Expressif.LanguageServer.Core.Tests.csproj --configuration Release --framework net10.0 --runtime win-x64 --no-restore --nologo --verbosity minimal -m:1 /nodeReuse:false (19 passed)
• dotnet test src/Expressif.LanguageServer.Tests/Expressif.LanguageServer.Tests.csproj --configuration Release --framework net10.0 --runtime win-x64 --no-restore --nologo --verbosity minimal -m:1 /nodeReuse:false (10 passed)
• git diff --check

Closes #16

[coderabbitai]

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