Native Rust editor responses ignore snippet and workspace-edit client capabilities #1516

Problem

P2; source-revalidated on main 3f2a835c78e9daba7baefc4c018c6fc9ca3c6e4a.

completion_config always enables SnippetCap and FillArguments; completion_item emits insertTextFormat=2 for snippets. workspace_edit always emits documentChanges, including create/rename operations. Gateway lsp_editor_request sends only method and params to the engine and forwards the result unchanged; it does not negotiate these response forms from initialize capabilities.

Location: crates/prod-code-engine-rust/src/editor/mapping.rs:25-39; crates/prod-code-engine-rust/src/editor/snapshot.rs:291-323.

Reproduction boundary

Initialize with capabilities={} and request function completion plus a text-only resolved assist. The paths can produce snippet placeholders and documentChanges although the client did not advertise support. A create/rename assist can additionally require unadvertised resourceOperations. Verify a minimal-capability client and a fully capable client separately.

These are source-derived predictions, not a newly executed LSP session or runtime result. Relevant paths are unchanged from the inspected audit checkout; no tests, builds or server restarts ran during this audit.

Expected behavior

Carry negotiated capabilities per editor session. Emit plain completion text when snippets are unsupported; use changes for text-only edits when documentChanges is unsupported, and do not offer unsupported resource operations. The official WorkspaceEdit specification defines these capability-dependent forms.

Original findings

• Rust completion, diagnostics, code actions and formatting for editors on prod-code lsp #320 (comment)
• Rust completion, diagnostics, code actions and formatting for editors on prod-code lsp #320 (comment)