fix(beta): omit compaction encrypted_content on request round-trip

[lllakshit]

Summary

• Strip encrypted_content from compaction blocks during request serialization so messages.append(response.content) no longer 400s with Extra inputs are not permitted.
• Add src/anthropic/lib/_compaction.py (non-generated) and hook it into transform / async_transform after Stainless TypedDict handling.
• Clarify compaction block docstrings and add respx-backed regression tests for dict/model round-trips, batch params, and web-search/advisor blocks that legitimately keep encrypted_content.

Fixes #1828.

Test plan

• uv run pytest tests/lib/test_compaction_request.py tests/lib/streaming/test_beta_messages.py tests/test_transform.py::test_transform_skipping -q
• uv run ruff check on changed files
• uv run pyright / uv run mypy on changed files

[lllakshit]

Hi @anthropics/sdk, this is a small request-side fix for #1828: compaction blocks include encrypted_content in responses, but sending it back on the next request 400s. The strip lives in src/anthropic/lib so it should survive Stainless regeneration, with regression tests for the round-trip and for blocks that should keep encrypted_content.

I just rebased onto current main. Happy to adjust if you want a different approach.