feat: add native PostgreSQL, SQL Server, and Oracle drivers

[openai0229]

Summary

• add PostgreSQL through tokio-postgres
• add SQL Server through tiberius
• add Oracle through pure-Rust oracle-rs without OCI, JDBC, or Java
• register all three in the unified NativeDriver SPI
• preserve existing managed JDBC datasource routing unless a native driver id is explicitly persisted
• add real-database product vertical tests and architecture documentation

Verification

• default toolchain: 357 passed, 9 ignored
• strict all-feature Clippy: passed
• Rust 1.88.0 check, tests, and strict Clippy: passed
• PostgreSQL 17 product vertical: passed with Java dormant
• SQL Server TDS product vertical: passed with Java dormant
• Oracle Free 23 smoke verticals: passed with Java dormant
• cargo fmt --check: passed
• git diff --check: passed
• knowledge lint: passed