test(postgres): make cancelled BEGIN regression deterministic

[shuvroroy]

Cancelling Pool::begin_with after PostgreSQL executes BEGIN must roll back before the backend is reused. The driver fix is already on main via #4394; this PR replaces its sleep-based regression test with the deterministic advisory-lock reproduction from #4423.

The test waits until pg_stat_activity confirms the backend is blocked after BEGIN, drops the pending future, and releases the lock. It then verifies that the same backend returns idle and that a subsequent pooled write survives a later transaction rollback. The synchronization works across supported runtimes without spawning a task.

Does your PR solve an issue?

Fixes #4423 by verifying the existing driver fix with the reported reproduction and strengthening regression coverage.

Is this a breaking change?

No. This only changes the PostgreSQL integration test.

Validation

Tested with PostgreSQL 17.11 and Rust 1.94.1:

• cargo test --no-default-features --features postgres,macros,runtime-tokio --test postgres --test postgres-error -- --skip test_pg_copy_chunked: 63 passed, 2 already ignored; the large COPY test is run separately by CI.
• The focused regression passes with Tokio, Smol, async-global-executor, and async-std.
• Temporarily restoring the pre-fix(postgres): roll back a transaction cancelled during BEGIN #4394 transaction-depth accounting makes the regression fail with idle in transaction instead of idle; restoring the existing fix makes it pass.
• cargo fmt --all -- --check passes.
• cargo clippy --no-default-features --features postgres,macros,runtime-tokio -- -D warnings passes.

Strict Clippy checks including --test postgres encounter existing warnings in sqlx-test/src/lib.rs and unchanged portions of tests/postgres/postgres.rs.