Revert new API and restore DBConnection

[ruslandoga]

What changed

• Revert new api #341 and restore the DBConnection-based public API on master.
• Keep compatible work landed after new api #341, including Ecto 3.14, current Decimal and Mint dependencies, RowBinary correctness fixes and optimizations, benchmark cleanup, and CI maintenance.
• Adapt the later RowBinary integration/property tests to the restored DBConnection query and result APIs.
• Keep the broader compatibility CI matrix, with explicit ClickHouse and timezone values.

Why

The replacement HTTP pool and breaking public API are not ready to ship yet. Restoring the released DBConnection architecture lets ch publish an Ecto 3.14-compatible release without forcing unrelated API, Elixir, and OTP migrations on users. Development of the new API can continue on a separate branch.

The revert was resolved against current history rather than resetting or rewriting master.

Verification

• Compared the result with v0.8.3; the DBConnection-era core files match that release, while the remaining diff consists only of intentionally retained maintenance, dependency, RowBinary, and test changes.
• Confirmed no NimblePool or new Help test-helper references remain.
• MIX_ENV=test mix format --check-formatted
• MIX_ENV=test mix compile --warnings-as-errors
• MIX_ENV=test mix test — 605 passed, 10 skipped, 5 excluded
• mix deps.unlock --check-unused
• git diff --check