fix: Make batch_add_requests split batches by serialized payload size

[vdusek]

The 9 MB payload guard in batch_add_requests was inert: constrained_batches was called without get_len, so the default len() measured each request dict's key count (~4) instead of its serialized size. Batches were therefore split only by the 25-request count limit, and large requests shipped as one oversized POST that the API rejects with 413, failing the whole call.

The guard now measures each request as its UTF-8 JSON byte length, using the same serialization flags as the HTTP client's request body path. It also passes strict=False, which preserves the previous contract for an individually oversized request: it's sent in its own batch and left for the API to judge, instead of raising a client-side ValueError. Both the size-based splitting and the oversized-singleton path are covered by new sync/async regression tests.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.64%. Comparing base (a8a393b) to head (2f90327).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #953      +/-   ##
==========================================
+ Coverage   94.61%   94.64%   +0.02%     
==========================================
  Files          58       58              
  Lines        5239     5263      +24     
==========================================
+ Hits         4957     4981      +24     
  Misses        282      282              

Flag	Coverage Δ
integration	92.28% <100.00%> (-0.39%)	⬇️
unit	83.94% <100.00%> (+0.20%)	⬆️

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[Pijukatel]

Looks good to me.

Claude had one more minor point:

Peak memory: requests_as_dicts and serialized_requests are now both alive for the whole call, so peak memory is roughly dicts + full serialized JSON at once. Previously the serialized form existed only per-batch at send time. For crawler-scale inputs (hundreds of thousands of requests) this is a real bump; a cheap mitigation is del requests_as_dicts after serialization. Acceptable tradeoff for exact measurement, but worth a thought.