fix(core): send logs and metrics through one OTLP batch sender (#4623)

* fix(core): retry logs batches after exhausted transient failures

Logs classified an exhausted 408/429/5xx as `fatal`, so the batch was dropped
instead of held for the next flush cycle. Fixes #4570.

* refactor(core): send logs and metrics through one OTLP batch sender

Collapses both senders into `_sendOtlpBatch` so the retry policy is one
decision, pins the classification with a per-signal test table, and retries
408 in the browser logs and metrics adapters.

* test(core): pin the shared OTLP retryCheck

The classification cases run with retries disabled, so nothing exercised the predicate that keeps 413 out of the retry loop.

* Repair PR #4623: fix(core): send logs and metrics through one OTLP batch sender

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