feat(core): expose acquired pool connections

[angelnereira]

Summary

• add Pool::num_acquired() to report checked-out pool connections
• include num_acquired in Pool debug output
• add coverage for the new counter through the Any pool tests
• close SQLCipher test connections explicitly so the SQLite matrix does not rely on process-exit teardown

This helps applications distinguish a full-but-idle pool from one where all connections are checked out, which is useful when diagnosing pool contention and PoolTimedOut errors.

The SQLCipher change is test-only. The SQLite matrix was reporting all sqlite-sqlcipher tests as passed and then segfaulting at process exit; closing the test connections explicitly makes the teardown deterministic.

Checks

• cargo fmt
• cargo check -p sqlx-core --no-default-features --features any,_rt-tokio
• cargo clippy -p sqlx-core --no-default-features --features any,_rt-tokio -- -D warnings
• cargo test --no-run --test any-pool --no-default-features --features any,sqlite,macros,runtime-tokio
• DATABASE_URL=sqlite::memory: cargo test --test any-pool pool_should_report_acquired_connections --no-default-features --features any,sqlite,macros,runtime-tokio
• RUSTFLAGS='--cfg sqlite_test_sqlcipher' DATABASE_URL=sqlite:tests/sqlite/sqlite.db cargo test --test sqlite-sqlcipher --no-default-features --features any,macros,migrate,sqlite,_unstable-all-types,runtime-tokio -- --test-threads=1
• RUSTFLAGS='--cfg sqlite_test_sqlcipher' DATABASE_URL=sqlite:tests/sqlite/sqlite.db cargo test --test sqlite-sqlcipher --no-default-features --features any,macros,migrate,sqlite,_unstable-all-types,runtime-smol -- --test-threads=1
• RUSTFLAGS='--cfg sqlite_test_sqlcipher' DATABASE_URL=sqlite:tests/sqlite/sqlite.db cargo test --no-default-features --features any,macros,migrate,sqlite,_unstable-all-types,runtime-tokio,sqlite-preupdate-hook -- --skip rustsec_2024_0363 --test-threads=1
• cargo test -p sqlx --doc --no-default-features --features any,postgres,macros,runtime-tokio
• git diff --check

Note: cargo clippy --test any-pool --no-default-features --features any,sqlite,macros,runtime-tokio -- -D warnings currently reports an existing clippy::needless_question_mark warning in sqlx-test/src/lib.rs, outside this change.