fix(bridge): gate capability-dependent LSP requests before dispatch

[bug-ops]

Summary

• Check the routed server's negotiated ServerCapabilities before dispatching capability-gated requests (rename, format_document, code_actions, call_hierarchy_prepare/incoming_calls/outgoing_calls, signature_help, inlay_hints, hover, definition, references, completions, document_symbols, workspace_symbol, implementation, type_definition), returning a new typed Error::CapabilityNotSupported { server_id, capability } instead of sending the request and opaquely relaying whatever the server does with an unsupported method. handle_diagnostics is deliberately left unguarded — it falls back to the push-notification cache on pull-request failure.
• The capability check runs before textDocument/didOpen is sent for gated handlers (new resolve_client_for_file/prepare_gated_document split).
• Extract the Result-to-JSON/McpError mapping repeated across all 20 tool handlers in mcp/server.rs into a single to_tool_result helper.

Closes #240
Closes #230

Test plan

• cargo +nightly fmt --all -- --check
• cargo clippy --all-targets --all-features --workspace -- -D warnings
• cargo nextest run --workspace --all-features --lib --bins (490 passed, 0 failed)
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features
• Blocked-path test for every one of the 16 gated handler call sites
• Positive-path test confirming dispatch proceeds when the capability is advertised
• Explicit Some(OneOf::Left(false)) (capability explicitly unsupported, not just absent) negative test

[Copilot]

Pull request overview

This PR strengthens the MCP↔LSP bridge by proactively gating capability-dependent LSP requests using the negotiated ServerCapabilities, and reduces boilerplate in MCP tool handlers by centralizing result-to-JSON/error mapping.

Changes:

• Add a new bridge-layer Error::CapabilityNotSupported { server_id, capability } and use it to gate capability-dependent LSP requests before dispatch (and before didOpen for document-opening tools).
• Refactor Translator’s dispatch preparation to split “resolve client/path” from “ensure document open”, enabling pre-open capability checks.
• Deduplicate MCP tool handler result mapping via a shared to_tool_result helper.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
crates/mcpls-core/src/mcp/server.rs	Introduces to_tool_result helper and updates tool handlers to use it.
crates/mcpls-core/src/lsp/lifecycle.rs	Adds a test-only LspServer::new_for_test fixture for capability-gate tests.
crates/mcpls-core/src/error.rs	Adds Error::CapabilityNotSupported and a display test.
crates/mcpls-core/src/bridge/translator.rs	Implements capability gating + pre-didOpen ordering and adds extensive tests.
CHANGELOG.md	Documents the new error and behavior changes under [Unreleased].