test(security): reproduce response-driven cookie persistence

[seonghobae]

Purpose

Close issue #134's response-driven cookie-state boundary without weakening explicit caller-owned cookie support. This PR remains intentionally stacked on #195 because the pinned public client must first stop self-denying HTTPX's ambient Connection default.

Exact current stack identity

• predecessor PR: fix(reliability): remove ambient HTTPX Connection default #195 (fix/public-client-default-headers-current-main);
• exact predecessor head: 5c4c06064f896338afbe5942f3a70f9eafe449f7;
• exact current test(security): reproduce response-driven cookie persistence #193 head: c2d3a004aaac06c570f32b6bca2d4ecb58452285;
• current base branch: fix/public-client-default-headers-current-main;
• current PR-base snapshot: 5c4c06064f896338afbe5942f3a70f9eafe449f7;
• test(security): reproduce response-driven cookie persistence #193 was non-destructively restacked with a two-parent merge preserving its four-path cookie-state delta while taking the exact current fix(reliability): remove ambient HTTPX Connection default #195 tree as the dependency side;
• keep Draft until fix(reliability): remove ambient HTTPX Connection default #195 is protected-main integrated and this PR is retargeted/refreshed onto the resulting live protected base.

Historical checks, reviews, comments, and generated merge revisions from earlier #193 or #195 heads are non-transferable.

Test-first security result

The regression proves that response-provided Set-Cookie remains visible to the caller without becoming ambient state on later requests from the same sync or async public client. The implemented narrow boundary uses a caller-owned CookieJar policy that rejects response-driven insertion while continuing to permit explicit caller cookie state. Exact (hostname, port) authorization and existing method, framing, redirect, proxy, Unix-socket, TLS, DNS, request/response resource and cleanup boundaries remain unchanged.

The current stack also preserves #195's removal of only HTTPX's ambient Connection default. Caller-supplied hop-by-hop fields remain subject to the existing fail-closed transport validator.

Exact-head evidence

On unchanged c2d3a004aaac06c570f32b6bca2d4ecb58452285:

• CI run 31590185081: completed success;
• no formal reviews existed at the last exact-head refetch;
• no unresolved review threads existed at the last exact-head refetch.

Because this is a non-default-base stacked Draft, this evidence is not merge authority and does not transfer after predecessor integration, retargeting, or any head movement.

Remaining acceptance

1. fix(reliability): remove ambient HTTPX Connection default #195 must reach protected main under its own exact-head/security/supply-chain/review gates.
2. Re-resolve protected main, retarget/reconstruct test(security): reproduce response-driven cookie persistence #193 non-destructively if needed, and regenerate every applicable CI/security/supply-chain/review gate on the resulting exact head.
3. Resolve only current valid findings; do not count predecessor, status-only, skipped, neutral, queued, rate-limited, wrapper-green, or model-only evidence as acceptance.
4. Merge only the unchanged accepted tree when live EgressWeave/CWL governance permits it.

Progresses #134; depends on #195.

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