test: add integration suite against real PostgreSQL and MySQL

[Puppo]

What

Adds a Docker-backed integration test suite that executes queries built by every public API of @nearform/sql against real PostgreSQL and MySQL. The existing tests only assert generated strings without ever running them, so regressions in placeholder numbering, identifier quoting, nested-statement offsets, or injection handling could pass CI while breaking real queries.

Coverage

11 cases × 3 drivers (pg, mysql2, legacy mysql) = 33 tests, all executed end-to-end:

• Basic parameterized insert/select round-trip
• Interpolated values stored as literals, not executed (injection)
• glue — IN clause + batch insert
• map — IN clause (default & object mapper)
• quoteIdent — dynamic table/column names + escaping the quote char on a live identifier
• unsafe — literal fragment interpolation
• Nested SqlStatement — proves $N placeholder numbering against a live server
• Deprecated append
• Null values
• Special characters / unicode round-trip

Changes

• integration/ — config + driver adapters (one query/raw/end interface) + a single dialect-aware feature suite shared by all three drivers.
• docker-compose.yml — add mysql:8.0 service with healthchecks, bump Postgres to 17-alpine.
• package.json — add mysql/mysql2 devDeps and db:up, db:down, test:integration, test:integration:docker scripts.
• .github/workflows/ci.yml — add MySQL service + integration test step.
• README.md — document the new commands.

Notes

• The legacy mysql driver can't speak MySQL 8's default caching_sha2_password, so the adapter uses a short mysql2 connection to switch the account to mysql_native_password first — works identically locally and in CI without a container command override (unsupported by GitHub service containers).
• Integration tests run with --test-concurrency=1 because both MySQL drivers share one sqlmap database; the pg suite is isolated in its own DB.

Verification

• All 33 integration tests pass against live Postgres + MySQL.
• npm test (unit) still passes and needs no DB; npm run lint clean.
• Regression check: deliberately broke the $N offset logic → nested-statement pg test failed as intended → reverted.

Run locally with npm run test:integration:docker.

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[Puppo]

Superseded by upstream PR nearform#236.