security: reject timeout-policy subclasses on the current policy stack

[seonghobae]

Purpose

Reconstruct the unique exact EgressTimeoutPolicy type boundary from stale/diverged Draft #158 on the current policy stack without transferring historical merge evidence.

This PR is intentionally stacked on #204 because both lanes modify src/egressweave/policy.py. Serializing them avoids parallel writers and lets the timeout boundary inherit the exact HTTP-method integrity tree rather than racing it.

Exact stack identity

• protected main ancestor: 7faf7a3b8a47980113982914000e724ab6a6cda5;
• predecessor security: reconstruct exact HTTP method strings on protected main #204 exact head/base: ab51ef1725bd20c898a3e3df82994b576e675e7b;
• RED-only head: a460ca5927c19ae593d70e508e7829dd772252ec;
• GREEN implementation head: 0b72ac4bd946f7a4aa4d6668f701859e03e45cdb;
• exact final head: b0e9b43d4b3c8701e8b6af7a93f1e82a0e9030c7;
• stale historical security: reconstruct exact request-timeout policy boundary on integrated main #158 head: 496d81709ad63be6b921510c447e7d2393daed96.

Test-first evidence

The RED-only head added two regressions requiring both public policy constructors to reject an EgressTimeoutPolicy subclass before a later transport can dynamically dispatch an overridden as_httpcore_timeout() implementation.

Hosted CI run 31783324371 failed on every Python 3.10–3.14 lane exactly because both new tests reported DID NOT RAISE TypeError; the Python 3.14 lane otherwise had 1,032 passing tests.

The minimal GREEN change replaces the subclass-accepting isinstance(...) check with an exact-type boundary. Follow-up commits document the pre-1.0 compatibility narrowing and add a documentation contract requiring the threat and migration path to remain explicit.

Implemented boundary

• require the exact built-in EgressTimeoutPolicy dataclass type during trusted EgressPolicy construction;
• reject subclass-controlled timeout exporters before transport dispatch;
• preserve exact instances, defaults, normalized authority and method policy, finite resource limits, deterministic evidence, and all public builder signatures;
• document that applications using subclasses must migrate to exact declarative instances;
• make no claim to sandbox arbitrary trusted Python code executing in the host process.

Exact-head verification

Exact head: b0e9b43d4b3c8701e8b6af7a93f1e82a0e9030c7

• CI succeeded on Python 3.10, 3.11, 3.12, 3.13, and 3.14.
• Python 3.14.6 lane: 1,035 tests passed.
• Production coverage: 1,762 / 1,762 statements and 590 / 590 branches (100%).
• Ruff, compileall, hourly product guard self-test, wheel/sdist build, archive verification, and installed-wheel smoke test succeeded.
• Full SAST and Security Scan workflows did not run for this stacked non-default-base PR; they remain mandatory after security: reconstruct exact HTTP method strings on protected main #204 integration and retargeting to protected main.

Merge contract

Keep Draft behind #204. Historical #158 checks, reviews, base identity, and mergeability are not acceptance evidence for this branch. Do not merge until:

1. security: reconstruct exact HTTP method strings on protected main #204 is integrated on an accepted protected tree;
2. this PR is retargeted/reconstructed on that exact tree without transferring historical evidence;
3. full current-head CI, SAST, substantive Dependency Review, Strix, and other required checks pass;
4. all current-head review findings are addressed and threads resolved;
5. independent approvals satisfy protected-branch policy; and
6. exact head/base/ancestry are revalidated immediately before merge.

Progresses #113. Supersedes #158 only after dependent #182 is safely retargeted to this replacement and #158 is closed as historical predecessor.

[coderabbitai]

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[seonghobae]

@coderabbitai review

Please review exact head b0e9b43d4b3c8701e8b6af7a93f1e82a0e9030c7, including the hosted RED failure on a460ca5927c19ae593d70e508e7829dd772252ec, the minimal exact-type GREEN change, migration documentation, and current 100% statement/branch coverage.

[seonghobae]

@opencode-agent

Please perform a fresh exact-head semantic review of b0e9b43d4b3c8701e8b6af7a93f1e82a0e9030c7. Validate the RED→GREEN lineage, exact-type threat model, compatibility/migration documentation, and the stacked-base merge contract. Submit a formal current-head verdict; do not treat the successful CI wrapper as a substitute for review.

[coderabbitai]

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[seonghobae]

@cwl-noema-review

Please independently review exact head b0e9b43d4b3c8701e8b6af7a93f1e82a0e9030c7 after a qualifying current-head OpenCode verdict. Confirm the exact-type restriction is proportionate, documented, and does not regress the public exact-instance policy path.