refactor(mcp): consolidate position params and centralize tool annotations

[bug-ops]

Summary

• Collapse six byte-identical position-only param wrapper structs (HoverParams, DefinitionParams, SignatureHelpParams, GoToImplementationParams, GoToTypeDefinitionParams, CallHierarchyPrepareParams) in mcp/tools.rs into direct use of PositionParams. rmcp's schema validation already strips the top-level title/description these wrappers carried before it reaches an MCP client, so the six contributed nothing beyond PositionParams itself.
• Replace the identical annotations(...) block repeated on all 20 #[tool(...)] handlers in mcp/server.rs with a single central pass in McplsServer::tool_router(), and collapse the redundant let result = { ... }; to_tool_result(result) pattern in 19 of 20 handlers to a single expression.
• A golden-snapshot test (tool_surface.json, captured from unmodified code) proves the client-visible tool surface (names, descriptions, titles, annotations for all 20 tools) is byte-identical before and after this refactor.

Closes #301, closes #302.

Design note

An earlier draft of this refactor proposed a macro_rules!-generated #[tool_router] block to also dedup the per-tool destructure/call/wrap boilerplate. That approach was withdrawn after adversarial review found a simpler mechanism (a ~10-line central annotations pass over the already-pub ToolRouter::map) achieves the same deduplication without introducing project-specific macro machinery, an unformattable hand-aligned table, or macro-hygiene footguns — consistent with this project's MVP/no-premature-abstraction convention. Note that neither approach closes #301's stated line/character argument-swap risk; that would require newtypes on the Translator::handle_* signatures, tracked as a separate follow-up.

Breaking change

mcpls_core::mcp::{HoverParams, DefinitionParams, CallHierarchyPrepareParams} are removed. Use mcpls_core::mcp::PositionParams directly. No deprecation shim, per this project's pre-1.0 policy.

Test plan

• cargo +nightly fmt --all -- --check
• cargo clippy --all-targets --all-features --workspace -- -D warnings
• cargo nextest run --workspace --all-features --lib --bins (643 passed, 1 intentionally skipped)
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --all-features
• Golden-snapshot test (test_tool_surface_matches_golden_snapshot) confirms zero client-visible change to the 20-tool MCP surface
• Independently re-verified against vendored rmcp-3.0.0/rmcp-macros-3.0.0 source (not just developer self-report) during review