refactor(language-server): pass LanguageId to Tool::run_diagnostic and variants

[Copilot]

Threads LanguageId through the Tool trait's diagnostic methods so tools can eventually use it for language-aware behavior. The language ID is currently ignored in all implementations — this is a pure refactor with no behavioral changes.

Changes

• Tool trait (tool.rs): Add language_id: &LanguageId parameter to run_diagnostic, run_diagnostic_on_save, run_diagnostic_on_change
• WorkspaceWorker (worker.rs): Thread language_id through collect_diagnostics_with and the three public run_diagnostic* methods
• Backend (backend.rs): Extract LanguageId from the in-memory file system (or request params in did_open) at each call site; restructured did_open to avoid an unnecessary language_id.clone()
• ServerLinter (server_linter.rs): Accept _language_id parameter (ignored for now)
• FakeTool / tests (tests.rs, worker.rs, tester.rs): Updated to match new signatures

// Before
fn run_diagnostic(&self, _uri: &Uri, _content: Option<&str>) -> DiagnosticResult;

// After
fn run_diagnostic(&self, _uri: &Uri, _language_id: &LanguageId, _content: Option<&str>) -> DiagnosticResult;

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