Fixing substring keyspace match issue.

[dtest]

Fixes #90

[kelp]

Nice fix — the two-layer approach (narrowed LIKE + exact-match filter in shardsForKeyspace) is the right shape. Worth noting the shardsForKeyspace exact-match is what actually makes it bulletproof: the LIKE "<db>/%" pattern alone still over-matches, since psc.Database is interpolated raw and _/% are LIKE wildcards (a keyspace named foo_bar would match fooXbar/...). The exact ks != keyspace check covers that, so the result is correct regardless. 👍

A couple of minor nits:

1. The removed // TODO: is there a prepared statement equivalent? is still valid. The query still interpolates psc.Database into a string literal, so the parameterization concern (and the LIKE-wildcard / unescaped-quote edge) hasn't gone away. Consider keeping the TODO, or binding the parameter:

   p.db.QueryContext(ctx, `show vitess_shards like ?`, psc.Database+"/%")

   That also closes the case where a keyspace name contains a ".

2. No integration coverage of the query change itself. The unit test exercises shardsForKeyspace but not the like "<db>/%" narrowing. Since test: add gated integration test suite #88 added the gated integration suite, a sibling-keyspace case there would lock in the half of the fix the unit test can't reach. Optional, given the defensive filter makes the result correct either way.