fix(mcp): serialize multiple databases in list_connections and add list_databases tool

[erneztox]

Hey! As discussed in the issue, here is the PR to improve the MCP server database discovery.

What changed?

1. Fix list_connections array serialization: In src-tauri/src/mcp/mod.rs, changed the database property extraction from .to_string() (which called .primary() and dropped items) to .as_vec(). Now the MCP client correctly sees all databases configured in a single connection.
2. Add list_databases tool: Added a native list_databases tool leveraging the existing driver.get_databases() trait. This allows the LLM to query and discover all available databases natively through the connection without needing to manually run SQL queries.

Testing

• Tested locally by passing JSON-RPC requests via stdio to the tabularis --mcp binary.
• Validated that list_connections outputs the full array.
• Validated that list_databases connects to the engine and returns the schemas correctly.
• Code compiles perfectly with cargo check.

Let me know if you need any adjustments!