fix(turso): unbound params executed as NULL — mysql CLI 'commands out of sync'

[luthermonson]

Problem

Oracle's mysql 8.4 CLI fails with ERROR 2014 (HY000): Commands out of sync on every statement (even SELECT 1) when the MySQL frontend is backed by litewire-turso. The same CLI works against the rusqlite backend, and PHP mysqli works against both.

Root cause

The mysql >= 8.1 CLI probes dollar-quoting support at startup by sending select $$ and expecting an ERR reply. Byte-level capture of both sessions showed the divergence:

• rusqlite: rusqlite rejects the bind-count mismatch ($$ parses as one SQLite parameter, zero provided) → ERR packet Wrong number of parameters passed to query. Got 0, needed 1 → CLI continues normally.
• turso: the Turso engine (0.7.0) executes statements with unbound parameters as NULL, so SELECT $$ returned a 1-column result set with a NULL row. The CLI only checks the probe's return code and never reads the result set, so libmysqlclient's state machine sticks at MYSQL_STATUS_GET_RESULT and every following mysql_real_query fails client-side with CR 2014. mysqli tolerated it because mysqlnd never sends this probe.

Fix

crates/litewire-turso/src/lib.rs: add an SQLite-tokenizer-style placeholder counter (?, ?NNN, :name, @name, $name; skips string literals, quoted identifiers, -- and /* */ comments) and reject bind-count mismatches in query()/execute() after prepare, with the same message shape rusqlite produces. turso 0.7.0 does not expose parameters_count() on its public Statement (it exists only on the private turso_sdk_kit inner), hence the scanner.

Tests

• param_count_scanner, unbound_parameter_rejected_like_rusqlite, execute_bind_count_mismatch_rejected (litewire-turso unit tests)
• crates/litewire/tests/mysql_turso_e2e.rs (feature turso): asserts select $$ yields a server ERR packet and the same connection stays usable, plus wire CRUD sanity
• cargo test --workspace: 25/25 suites pass; cargo clippy --workspace --all-targets -- -D warnings and cargo clippy -p litewire --features turso --all-targets -- -D warnings clean; cargo fmt --check clean

Repro evidence (mysql:8.4 CLI via podman)

Before: ERROR 2014 (HY000) at line 1: Commands out of sync; you can't run this command now
After:

$ mysql -P 33071 -e "SELECT 1"                      # turso backend
1
$ mysql -P 33071 -e "CREATE TABLE t ...; INSERT ...; SELECT * FROM t; SHOW TABLES;"
id  v
1   x
Tables_in_database
t

rusqlite backend behavior unchanged.