fix(rqlite): validate rqlite node before first query

[doublesilver]

Changes

Before executing any SQL against an rqlite endpoint, perform a lightweight
probe: GET / on the configured host and check for the X-Rqlite-Version
response header. If the header is absent the server is not an rqlite node
and an informative error is raised immediately — before any query is sent.

How it works

• RqliteQueryable gains a testConnection() async method that issues
  GET / with redirect: "manual" (so it works whether the server
  redirects to /status or not).
• A connectionVerified flag ensures the check runs only once per
  instance; subsequent transaction() calls skip straight to the query.
• On network failure the error message includes the configured endpoint
  to make debugging straightforward.

This follows the approach recommended in the issue by the rqlite
maintainer: call / and look for X-Rqlite-Version in the headers
(value irrelevant, presence is sufficient).

Testing

Unit tests added in src/drivers/database/rqlite.test.ts:

• testConnection resolves when X-Rqlite-Version header is present
• testConnection throws with a clear message when the header is absent
• testConnection wraps network errors with the endpoint URL
• transaction calls testConnection on the first invocation
• transaction skips testConnection on subsequent invocations
• transaction injects the Basic auth header when credentials are provided
• transformRawResult maps columns/values and handles statement-only results

All 101 tests pass (npm test). TypeScript strict check clean (npm run tsc).

/claim #59