Close test coverage gaps identified in the 0.2.0 test-suite audit

[CaptainDriftwood]

Context

A coverage-backed audit of the suite (511 unit + 64 integration tests) found the following untested areas, ranked by importance.

Tier 1 — untested code guarding correctness/security

• IcapResponse.encapsulated (response.py:236-261): the cached property is never accessed by any test — not the parse path, the None branch, caching, or IcapProtocolError propagation on a malformed header. Only EncapsulatedParts.parse is fuzzed directly.
• Async max_response_size enforcement (async_icap.py:805/885/949): Content-Length-exceeds, chunk-exceeds, and cumulative-body-exceeds are tested sync-only (test_client_methods.py:1309-1360); mirror them for AsyncIcapClient.
• Trailer flood without CRLF (icap.py:860 + async twin): the trailer cap's no-CRLF-flood branch is untested (existing tests only exercise the per-line cap path). Also uncovered: connection-close mid-chunk-size-line (icap.py:841), mid-trailer (icap.py:866), and mid-headers (icap.py:682).
• Async preview/stream error paths: _send_with_preview not-connected / timeout / CancelledError / OSError / early-virus-200, and _scan_stream_chunked CancelledError + 5xx branches — all tested sync, none async.

Tier 2 — parity and API-contract holes

• Sync __exit__ during an exception (async version is tested; sync only has a structural hasattr check).
• Async reqmod unit tests for body / custom headers / auto-connect (sync has all; async has only basic).
• port setter range ValueError untested on both clients; MockIcapClient.port setter lacks range validation entirely (mock/real parity break).
• Back-compat import test for icap.pytest_plugin.mock — the re-export module is the compatibility promise of the plugin split, and no test imports it (0% coverage).
• Minor: istag unquoted-value branch (response.py:302), CaseInsensitiveDict.__contains__ non-str key (response.py:124), EncapsulatedParts req_body/null_body/opt_body concrete assertions, 204-direct-reply-to-partial-preview flow.

Line numbers reference the improve/code-quality branch at the time of the audit.