Fix #890: never emit documentSymbol entries with empty names

[DavidObando]

Fixes #890

Root cause

The VSCode extension repeatedly logged:

[Error] Request textDocument/documentSymbol failed.
Error: name must not be falsy

SyntaxToken.IsMissing is defined as Text == null, and the parser's
MatchToken returns a token with null Text whenever an expected token is
missing. For incomplete or error declarations — e.g. func/struct/enum
with no name, or let = 42 — the identifier token is synthesized with a
null Text.

DocumentSymbolComputer.ComputeDocumentSymbols copied Identifier.Text
straight into DocumentSymbol.Name, so the textDocument/documentSymbol
response contained symbols whose name was null/empty (falsy). The
vscode-languageclient asDocumentSymbols protocol converter validates that
every symbol has a non-empty name and throws "name must not be falsy",
which breaks the Outline view and breadcrumbs for the affected file.

Fix

Guard every symbol-construction site in DocumentSymbolComputer with a new
SymbolName(SyntaxToken identifier, string fallback) helper that substitutes a
sensible, non-empty fallback name whenever the identifier token is missing,
empty, or whitespace:

• <function>, <variable>, <struct>, <enum> for top-level declarations
• <field>, <member> for struct fields and enum members (child symbols)

This covers every node kind the computer can emit (functions, global
variables, structs + their fields, enums + their members), so a null/empty
name can never reach the protocol layer. The fix is in the language server
source, not the bundled extension artifact.

Tests

Added regression tests in DocumentSymbolHandlerTests that:

• assert no returned DocumentSymbol (or any child) ever has an empty/null
  name for a range of incomplete declarations, and
• confirm valid symbols still appear with their correct names alongside
  incomplete ones.

Local build + test results

• dotnet build GSharp.sln -c Release --no-restore -graph → succeeded, 0 errors
• dotnet test GSharp.sln -c Release --no-build → all green:
  • LanguageServer.Tests: 335 passed
  • Core.Tests: 3340 passed (1 skipped)
  • Compiler.Tests: 1347 passed
  • Interpreter.Tests: 308 passed
  • Extensions.Tests: 104 passed
  • Sdk.Tests: 38 passed
  • 0 failures

Files changed

• src/LanguageServer/HoverComputer.cs — DocumentSymbolComputer fix
• test/LanguageServer.Tests/DocumentSymbolHandlerTests.cs — regression tests