Fix substring keyspace match in shard discovery

[rpatel27]

Fixes a confirmed bug reported by Trengo (PlanetScale ticket #37380). This ports the same three fixes already landed on airbyte-source in planetscale/airbyte-source#158.

Every Singer sync against the trengo database logged one ERROR per stream:

shard provided in VGTID, trengo_etl/-, not found in the trengo keyspace

The real trengo/- data still synced, and the job succeeded, because cursor-peek failures wrapped a nil err.

Root cause

1. GetVitessShards ran show vitess_shards like "%trengo%" and TrimPrefix(name, "trengo/"). The sibling keyspace trengo_etl matches the LIKE; because the row does not start with trengo/, TrimPrefix left trengo_etl/- in every stream's saved state.
2. In Read, when cursor peek failed (lcErr), the code did errors.Wrap(err, ...) while err was still nil, so the ERROR was logged but the shard counted as synced.
3. Fixing discovery alone would leave the stale trengo_etl/- key in saved state. Once the nil-error bug is fixed, those streams would start failing.

This matches the existing fivetran-source #90 fix and airbyte-source #158 fix (GetVitessShards + shardsForKeyspace).

Changes

• Discovery uses show vitess_shards like "<keyspace>/%" and parses with an exact keyspace cut (shardsForKeyspace), so sibling keyspaces that share a prefix cannot leak in.
• Cursor-peek failures return a non-nil error (errors.Wrap(lcErr, ...) / errors.New for an empty stop position).
• When loading/merging sync state, drop shard keys that current discovery no longer returns and seed empty cursors for newly discovered shards, keeping valid cursors for real shards.

Test plan

• TestShardsForKeyspace_SiblingKeyspacePrefix — trengo vs trengo_etl, plus a sharded test / test_sharded case
• TestSync_PrunesStaleSiblingKeyspaceShards — drops trengo_etl/-, keeps the real trengo/- cursor, seeds empty cursors for newly discovered shards
• TestRead_ReturnsErrorWhenCursorPeekFails — peek Recv error is returned instead of a successful nil wrap
• TestRead_ReturnsErrorWhenCursorPeekStopPositionEmpty — empty stop position returns a non-nil error
• go test ./... passes

References: planetscale/airbyte-source#158, Trengo / PlanetScale ticket #37380