sync_turn() sent the whole turn to backend.add() untruncated. OSS
embedding models with small context windows (Ollama bge-small-zh-v1.5:
512 tokens) reject the request with HTTP 500, and hosted APIs answer
INPUT_TOKEN_LIMIT_EXCEEDED — in both cases _try() only logs, silently
dropping the turn's memory extraction after long conversations.
Cap each synced message at its last sentence boundary within 450 chars
before ingestion: short turns pass through unchanged, long turns keep a
coherent statement for fact extraction. This replaces the previous
retry-on-error approach, which could not match Ollama's HTTP 500 shape.
Salvaged from #37427 with the test suite trimmed to the one invariant
(oversized turn still reaches a small-context backend, short text
untouched, no breaker failure).
Refs #37421#106235
Co-authored-by: szicely <140148567+szicely@users.noreply.github.com>
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