aws_sqs: enforce 256 KB message and batch size limits

[twmb]

Reject individual messages exceeding the SQS 256 KB limit before sending, and split batches by cumulative byte size so that each SendMessageBatch call stays within the 256 KB total payload limit.

Closes #756

[claude]

Claude finished @twmb's task in 9m 6s —— View job

---

Review: aws_sqs: enforce 256 KB message and batch size limits

• Gather context and read project guidelines
• Validate commit policy
• Review code changes
• Post summary

---

Commits
LGTM

Review
Solid implementation. The sqsEntrySize function correctly accounts for what SQS counts toward the 256 KB limit (body + attribute names + attribute string values + data types), and the comment explicitly documents why only StringValue is counted. The batch-splitting loop's n > 0 guard is safe because oversized individual messages are already rejected before the loop. The byte-size splitting composes correctly with the count-based splitting in writeChunk. Tests cover the key cases: entry size calculation, oversized single message rejection, and byte-aware batch splitting.

LGTM

[josephwoodward]

Do we still need to be concerned about @Jeffail's comment in the original issue?

[mmatczuk]

@twmb please rebase

[twmb]

@josephwoodward

"""
Regarding Jeffail's concern: he worried about "odd side effects if we change the default batching config to something non-zeroed" but said "Even if we can't do this for whatever reason we can enforce it at the edge so at the very least we won't receive errors."

This PR does exactly the safe approach he suggested — it does not change any default batching config. It enforces limits at the edge in WriteBatch:

1. Rejects individual messages > 256 KB before they reach SQS
2. Splits batches by cumulative byte size so each SendMessageBatch call stays within the 256 KB total payload limit

The batch policy (NewBatchPolicyField) is completely untouched — users still configure their own batching as before. The byte-size splitting is purely a safety net at send time. So Jeffail's concern doesn't apply here.
"""

[claude]

Commits
LGTM

Review
Adds SQS 256 KB message and batch size enforcement with byte-aware chunking before writeChunk. Individual oversized messages are rejected early, and multi-message batches are split to stay within the 256 KB total payload limit. Good test coverage for size calculation, rejection, and splitting.

LGTM