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!

[debba]

Hey, thanks a lot for this! I reviewed it locally and I'm happy with it — the fix is correct and the new tool follows the existing patterns nicely.

I verified everything end-to-end against the demo stack (demo/docker-compose.yml), driving tabularis --mcp over stdio:

• tools/list advertises list_databases alongside the existing tools ✅
• list_connections now returns the full array — the demo MySQL connection correctly shows ["tabularis_demo","blog_demo","perf_demo"] instead of just the first one ✅
• list_databases returns the real database list on both MySQL and Postgres ✅
• Missing connection_id → -32602, unknown connection → -32000 ✅
• The audit trail (ai_activity.jsonl) records the new tool with correct row counts and error statuses ✅

cargo check is clean and the existing test suites pass on the branch.

A few small things I noticed — none of them blocking, but since they're all quick I'd prefer to get them into this PR rather than leaving them behind:

1. README tools table — the MCP tools table in README.md (and the translated READMEs) doesn't list list_databases yet.
2. AI Activity filter — the tool filter dropdown in src/components/settings/ai-activity/AiActivityEventsTab.tsx hard-codes the tool names, so list_databases events show up in the list but can't be filtered. Adding it to toolOptions (and to the AiToolName union in src/types/ai.ts for completeness) would be great.
3. A small test — a dispatch-level test for the new tool (e.g. missing/unknown connection_id → the right JSON-RPC errors) would be nice to have per our testing conventions.

One unrelated thing my testing surfaced: keychain_utils.rs uses println!, so [Keychain] ... log lines end up on stdout inside the MCP JSON-RPC stream. That's pre-existing (it happens on main too), so no need to touch it here — I'll open a separate issue for it.

Thanks again! 🙏