refactor(body_cap): share non-iteration logic between sync/async

[lesnik512]

Summary

• Extracts two byte-identical blocks from _read_capped/_read_capped_async in _internal/body_cap.py into shared functions: _reject_if_declared_length_exceeds_cap (declared-Content-Length early-reject) and _build_buffered_response (final Response reconstruction).
• The byte-accumulation loop stays duplicated — sync/async genuinely differ there, same judgment applied to every other sync/async split this session.
• Closes out the deepening of the body_cap module that this session's first refactor extracted out of client.py but didn't fully share at the time.

Fix folded in from final review: no test anywhere asserted content_length on the reason="streamed" path when a declared length is actually present (the gzip-bomb scenario) — every existing "streamed" test used a chunked body with no declared length. A dropped return value across the new function boundary would have shipped green at 100% coverage. Added the missing assertion plus a new async mirror test (also closing a pre-existing sync/async parity gap — there was no async gzip-bomb test at all before this).

Change file (Lightweight lane): planning/changes/2026-07-13.06-body-cap-shared-non-iteration-logic.md

Test plan

• just test — 780 passed, 100% coverage
• just lint-ci — ruff format/check, ty check, planning validator all clean
• Task-scoped review (spec + quality): Approved, zero findings
• Final whole-branch review: Ready to merge = Yes after one fix cycle (test-gap finding empirically verified as closed)