IteratorByteStream: split large iterator chunks into 64 KB pieces

[Treadgold]

Summary

Split large iterator chunks in IteratorByteStream into 64KB pieces to prevent memory spikes when sending large content streams.
This ensures that very large generator chunks are safely split into manageable sizes internally without changing the public API.
A test was added in tests/test_content.py to verify that large chunks are split correctly.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.(No Public docs changes were needed.)

Closes #3660

[Treadgold]

Just realized there’s an existing PR (#3661) for this — completely happy for this one to be closed if maintainers prefer theirs, I should have checked before starting.