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fix: address PR review on the webhook ndjson format

- Normalize the stored header keys in `transform_config/1` before the
  content-type is added. A config that holds "Content-Type" no longer ships a
  duplicate header.
- Pattern match on the ndjson format instead of an `if` in `transform_config/1`.
- Reuse `NdjsonFormatter.encode/1` in place of a second implementation.
- Extract `format_payload/2`, shared by `test_connection/1` and
  `Pipeline.handle_batch/4`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UeRcBGtdgk6tUX5gJohbTB