[shuvroroy]

Connection::transaction rejects callbacks that capture Send but non-Sync values such as std::sync::mpsc::Receiver. The callback is moved into the returned future and invoked once, so this removes the unnecessary Sync bound while retaining Send.

Add a shared regression helper to the native PostgreSQL, MySQL/MariaDB, SQLite, and Any suites. It holds a receiver across an await, checks that the transaction future is Send, and verifies commit on Ok and rollback on Err.

Does your PR solve an issue?

Fixes #4513.

Is this a breaking change?

Yes, for custom Connection implementations that override transaction with the old F: Sync bound: they must relax that bound to match the trait. Existing callers remain compatible. All in-tree drivers inherit the default implementation.

Validation

• The shared regression fails to compile with the original Sync bound and compiles after its removal.
• All eight native/Any regression runs passed: PostgreSQL 17.11, MySQL 8.4.13, MariaDB 11.4.13, and SQLite.
• SQLite integration suite: 45 passed, one existing ignored test.
• Core unit tests with --all-features: 11 passed.
• All-driver library Clippy with -D warnings, cargo check -p sqlx-test, formatting, and git diff --check passed.