fix(rqlite): validate node via X-Rqlite-Version header before first query

[mustafash0099]

Problem

When connecting to an rqlite endpoint, Outerbase Studio would fail or return confusing errors if the configured URL pointed to a non-rqlite server (wrong host, wrong port, or misconfigured proxy).

Fixes #59

Solution

Before executing any SQL, RqliteQueryable now probes the configured endpoint with GET / and verifies the X-Rqlite-Version response header is present, as recommended by the rqlite maintainer in the issue and the rqlite CLI source.

Changes

src/drivers/database/rqlite.ts

• Added testConnection(): Promise method that calls GET / with redirect: manual
• Throws a descriptive error when the X-Rqlite-Version header is absent or the host is unreachable
• Auth credentials are excluded from the probe (the rqlite root path is a public health-check endpoint)
• transaction() calls testConnection() on first use; result is cached so no overhead on subsequent calls

src/drivers/database/rqlite.test.ts (new)

• transformRawResult: column mapping, empty rows, duplicate column renaming
• testConnection: resolves on valid header, throws on missing header, throws on network error, does NOT send auth during probe
• transaction: validates before first query, skips on subsequent calls, propagates probe failure, sends auth on query requests
• query: validates and returns first result set

Test plan

• Run npm test -- all new tests pass, no existing tests broken
• Connect to a valid rqlite node -- queries execute as before
• Enter a wrong host/port -- clear error: Cannot reach rqlite at ...
• Enter a non-rqlite HTTP server -- clear error: does not appear to be an rqlite node

/claim #59