Language-feature requests: typed params/results + request-response correlation

[christiankissig]

Implements #13 — the general LSP pieces downstream clients need for language features (hover, completion, definition, documentSymbol, documentHighlight).

1. Request/response correlation (client.py)

• LSPClient tracks in-flight requests in _pending_requests: dict[id, asyncio.Future].
• New request(request, timeout=...) coroutine sends a request and awaits its result, correlating the response by id. Raises LSPError on a ResponseError; on timeout it fires $/cancelRequest and raises asyncio.TimeoutError.
• New cancel_request(id) sends a spec-correct $/cancelRequest notification (no top-level id).
• _handle_response resolves/rejects the matching future and still forwards server-initiated requests, notifications, and unmatched responses to the existing response_handler.
• Default timeout configurable via request_timeout on __init__ / from_command. send_request stays fire-and-forget for backward compatibility.

2. Request types + typed params (protocol.py)

• New DocumentSymbolRequest and DocumentHighlightRequest.
• Hover/Completion/Definition (and the two new requests) accept a typed params model or a raw dict, via a shared _coerce_params helper — existing dict-based callers are unaffected.
• CompletionParams with optional CompletionContext { triggerKind, triggerCharacter? }.

3. Result models + union parsers (protocol.py)

• Hover: Hover, MarkupContent, MarkupKind, MarkedString.
• Completion: CompletionList, CompletionItem, CompletionItemKind, InsertTextFormat, TextEdit, InsertReplaceEdit, CompletionTriggerKind.
• Definition: LocationLink (alongside existing Location).
• DocumentSymbol: DocumentSymbol (recursive), SymbolInformation, SymbolKind, SymbolTag.
• DocumentHighlight: DocumentHighlight, DocumentHighlightKind.
• Five parse_*_result helpers that validate and normalise each runtime union (bare CompletionItem[] → CompletionList, single Location → [Location], LocationLink vs Location by targetUri, DocumentSymbol vs SymbolInformation by location).

All new names are exported from lsp_client/__init__.py.

Acceptance

• ✅ A caller can issue each request and await a typed, validated result (or None).
• ✅ Result-union parsing helpers for definition/completion/documentSymbol (+ hover/highlight).
• ✅ Correlation covered by tests: resolve-by-id, error propagation, timeout+cancel, default timeout, unmatched-response passthrough.

Checks

• poetry run pytest — 101 passed
• poetry run ruff check . / ruff format --check . — clean
• poetry run mypy . — clean

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