feat: add MySQL/Redis Wasm integration tests (US-117)

Implement end-to-end integration tests proving Wasm guest components
can communicate with MySQL and Redis through WarpGrid's database proxy.

New test file: integration_wasm_mysql_redis.rs (gated behind
cfg(feature = "integration")) with 6 tests:

- test_mysql_crud_via_wasm: CREATE TABLE → INSERT → SELECT → DROP TABLE
- test_redis_crud_via_wasm: SET → GET → DEL
- test_mysql_pool_reuse_via_wasm: connect → query → close → reconnect → query
- test_redis_pool_reuse_via_wasm: connect → SET → close → reconnect → GET
- test_mysql_health_check_removes_dead: COM_PING detects dead connections
- test_redis_health_check_removes_dead: PING detects dead connections

Includes StatefulMockMysqlServer (SQL-aware query dispatch with state
tracking) and StatefulMockRedisServer (RESP protocol with HashMap
key-value state). All clippy warnings resolved.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: nadilas