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.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.60%. Comparing base (a8a393b) to head (01494f4).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #953      +/-   ##
==========================================
- Coverage   94.61%   94.60%   -0.02%     
==========================================
  Files          58       58              
  Lines        5239     5242       +3     
==========================================
+ Hits         4957     4959       +2     
- Misses        282      283       +1     

Flag	Coverage Δ
integration	92.40% <100.00%> (-0.27%)	⬇️
unit	83.74% <100.00%> (+<0.01%)	⬆️

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