reliability: public pinned clients must not self-deny HTTPX default Connection header

[seonghobae]

Buyer-visible reliability defect

Protected main remains 7faf7a3b8a47980113982914000e724ab6a6cda5. The defect is reproduced and the bounded source repair is implemented on Draft PR #195, but it is not protected-main truth yet.

HTTPX adds an ambient Connection client default. EgressWeave correctly rejects hop-by-hop request metadata at its transport boundary, so before the repair an ordinary policy-allowed request built through the public EgressWeave client could deny itself before reaching the validated pinned pool unless the caller manually removed that framework default.

The root-cause remedy belongs at EgressWeave client construction, not in the transport validator: remove only HTTPX's ambient Connection default while continuing to fail closed on caller-supplied hop-by-hop fields.

Exact current implementation lane

Fresh identity:

• protected/live base: 7faf7a3b8a47980113982914000e724ab6a6cda5;
• canonical PR: fix(reliability): remove ambient HTTPX Connection default #195, fix(reliability): remove ambient HTTPX Connection default;
• exact current PR head: 5abc442bb4269c2bdce74cd870bfc9889c70be89;
• state: open / Draft / mergeable;
• ancestry: 6 commits ahead / 0 behind exact protected main;
• scope: src/egressweave/sync_transport.py, src/egressweave/transport.py, and tests/test_public_client_default_headers.py.

The current tree removes only the ambient HTTPX Connection default during synchronous/asynchronous public client construction. Caller-supplied hop-by-hop fields remain rejected by the existing fail-closed request-header validator. Destination authorization, normalized (hostname, port), methods, target/framing controls, redirects, proxies, Unix sockets, TLS identity, DNS pinning/revalidation, explicit credentials, finite resource budgets and cleanup are unchanged.

This PR remains the required predecessor for #193 / #134 response-cookie-state work.

Exact-current-head evidence

On unchanged 5abc442bb4269c2bdce74cd870bfc9889c70be89:

• repository CI 31596370532: success;
• SAST Semgrep 31596370551: success;
• Security Scan 31596370786: aggregate success, but dependency-review job 94112774879 skipped the actual immutable-pinned Dependency review action; wrapper green is therefore non-passing supply-chain evidence;
• OpenCode check 94121293201: success as check/model evidence only;
• Noema check 94112774229: success as separate machine/model evidence;
• Strix exact-head check 94112774246: wrapper success with Strix backend unavailable and no vulnerability report; this is non-passing semantic security evidence;
• formal reviews: none;
• unresolved review threads: none.

No wrapper, skipped action, backend-unavailable result, status/model evidence, predecessor head or synthetic merge revision is promoted into acceptance.

Acceptance criteria

• Add focused sync and async public-builder RED tests proving an ordinary allowed request must reach an injected pinned pool without caller deletion of framework defaults.
• Prove the final low-level request omits the ambient Connection hop-by-hop default.
• Preserve fail-closed rejection of caller-supplied hop-by-hop headers; do not weaken _build_safe_request_headers.
• Remove only the conflicting ambient client default without widening destinations, methods, redirects, proxies, Unix sockets, TLS identity, DNS pinning, framing, credentials or resource limits.
• Keep synchronous and asynchronous public-builder behavior aligned.
• Obtain exact-head repository CI and SAST success on the current repair.
• Integrate the separately governed organization Dependency Review fail-closed repair and obtain fresh unchanged-head Security Scan evidence where the actual immutable-pinned Dependency review action executes and succeeds.
• Obtain an actual exact-head Strix semantic report rather than backend-unavailable/no-report/wrapper-only evidence.
• Revalidate every current human/automated finding, live base, ancestry, ruleset, required workflows and review threads immediately before integration.
• Merge only the unchanged accepted tree under live governance and verify the ordinary public-client request path from protected main.
• Only after fix(reliability): remove ambient HTTPX Connection default #195 reaches protected main, retarget/reconstruct test(security): reproduce response-driven cookie persistence #193 on the resulting exact protected base, reproduce security: prevent response-driven cookie persistence across egress requests #134 at the intended response-cookie boundary, and regenerate every base-sensitive evidence channel.

RCA discipline / non-goals

Do not solve this by allowing Connection: keep-alive through the security transport, by churning the clean #195 head to retrigger external providers, or by adding an EgressWeave-local substitute for organization-owned Dependency Review or Strix. The strict transport-side hop-by-hop rejection remains the correct protocol/security boundary.