Map internal symbols to correct LSP symbol and completion kinds

[Widthdom]

Summary

LSP kind mapping is incomplete: imports appear as Variable, while constructors appear as Function in both symbol and completion paths.

Audit IDs: G-24 and G-25. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Follow-up to closed #1762's SymbolKind mapping work.

Expected behavior and PR scope

One exhaustive, documented mapping should serve document/workspace symbols and completion, with deliberate fallbacks. This is one protocol-kind mapping/test-matrix PR.

Implementation guide for Codex 5.6 sol xhigh

• Create a table from every internal symbol kind to LSP SymbolKind and CompletionItemKind.
• Use Constructor for constructors and the correct namespace/module representation for imports under the supported protocol.
• Cover classes, records, interfaces, enums/members, methods, properties, fields, events, parameters, aliases, namespaces, and unknown/plugin kinds.
• Assert mappings from decoded protocol responses, not only helper return values.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• Imports no longer appear as variables.
• Constructors use Constructor in navigation/symbol and completion contexts.
• Every supported internal kind has tested, intentional mappings.

[Widthdom]

Work has started on branch fix-issue4870.