♻️ Migrate MCP server to rmcp framework

[renardeinside]

Summary

• Migrate from hand-rolled MCP server to the rmcp framework — replaces custom JSON-RPC transport, message parsing, and routing with rmcp's #[tool_router] / #[tool_handler] macros and stdio transport
• Restructure monolithic core.rs into modular files — context.rs, indexing.rs, info_content.rs, resources.rs, validation.rs, and tools/ submodules (databricks, devserver, docs, project, registry)
• Code quality cleanup — deduplicate AppPathArgs, use rmcp's native structured_error(), fix server name reporting ("apx" instead of "apx-mcp"), add missing read_only_hint annotation on databricks_apps_logs, remove unused reqwest dependency, canonicalize validated paths, add validation tests, add explicit shutdown signaling for background tasks

Test plan

• cargo build -p apx-mcp — compiles cleanly
• cargo test -p apx-mcp — all 15 tests pass (including 4 new validation tests)
• cargo clippy -p apx-mcp — no warnings
• cargo build — full workspace builds

🤖 Generated with Claude Code