enhancement(opentelemetry sink): warn on invalid json batching (#25423)

* enhancement(opentelemetry sink): warn on invalid json batching

Detect when the sink is built with `encoding.codec: json` and
`batch.max_events` greater than 1, which produces newline-delimited JSON
request bodies that OTLP receivers reject with HTTP 400. Log a warning
at startup pointing users at either `encoding.codec: otlp` (recommended)
or `batch.max_events: 1`. Also add a warning callout on the sink docs
page so the pitfall surfaces above the configuration reference.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* chore: drop changelog fragment

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>