Targeted subset of staging commit cfcdf8714a (#30202): only the
anthropic_passthrough_logging_handler.py hardening hunks and their four
tests are taken; the rest of that staging batch is intentionally excluded.
Backport adaptation (stable/1.88.x): the new TestBuildCompleteStreamingResponseRobustness
class is unioned with this line's existing test_parity_*/test_collapse_* tests, which
the 1.84.x base of this commit did not have. Source handler applies verbatim.
(cherry picked from commit cfcdf8714a)
(cherry picked from commit 973c7eb8d6)
f59192b87a