Fix panic in InitThriftClient when endpoint URL is malformed

[gretasharoyan]

This surfaces as:

interface conversion: thrift.TTransport is nil, not *thrift.THttpClient

Root cause

In internal/client/client.go, the HTTP transport path type-asserts the transport before checking the error returned by thrift.NewTHttpClientWithOptions:

tTrans, err = thrift.NewTHttpClientWithOptions(endpoint, thrift.THttpClientOptions{Client: httpclient})

thriftHttpClient := tTrans.(*thrift.THttpClient)   // panics
...
if err != nil {                                    // never reached on failure
    return nil, dbsqlerrint.NewRequestError(context.TODO(), dbsqlerr.ErrInvalidURL, err)
}

NewTHttpClientWithOptions returns a nil TTransport when url.Parse(endpoint) fails (e.g. a host or HTTP path containing a control character or an invalid %-escape). Asserting a nil TTransport to *thrift.THttpClient panics, so the existing err check below the switch is never reached.

Because the transport is initialized lazily on the first connection (connector.Connect → InitThriftClient), this surfaces as a crash while acquiring a connection rather than a returned error.

Fix

Move the error check to immediately after NewTHttpClientWithOptions, before the type assertion. The now-unreachable post-switch error check is removed; the same ErrInvalidURL error is returned as before, without panicking.

Testing

Added TestInitThriftClientMalformedEndpointDoesNotPanic, which builds a config whose HTTP path passes ToEndpointURL's non-empty validation but fails url.Parse, and asserts InitThriftClient returns an error instead of panicking. Verified the test fails (panics) without the fix and passes with it. go vet and gofmt are clean and the full internal/client package tests pass.