refactor(subscriber): make OutboxSubscriberConfig validate itself

[lesnik512]

Summary

Subscriber-knob validation lived in subscriber/factory.py::_validate_subscriber_config, called from create_subscriber. OutboxSubscriberConfig was a bare @dataclass with no __post_init__, so constructing it directly bypassed every guard. This moves validation into OutboxSubscriberConfig.__post_init__ so every construction path — @broker.subscriber, @router.subscriber, direct construction — is validated; there's no way in that skips it. The factory shrinks to construction/wiring.

Candidate #3 from the 2026-06-23 architecture review. Change record: planning/changes/2026-06-23.03-self-validating-subscriber-config/change.md.

What changed

• OutboxSubscriberConfig.__post_init__ → guarded super().__post_init__() (upstream SubscriberUsecaseConfig has none today, but dataclasses call only the most-derived one — the guard keeps a future upstream init running) then self._validate().
• _validate() holds the relocated checks. Behavior preserved exactly: maps _ack_policy is EMPTY → None so checks match on the explicitly-passed policy as before (the NACK+NoRetry advisory still fires only on an explicit NACK_ON_ERROR, not on the default that resolves to it).
• factory.py drops _validate_subscriber_config and its call.

Version-robustness wrinkle (the interesting part)

Moving validation under the dataclass-generated __init__ inserts a frame whose co_filename is the literal "<string>" between the user's call and __post_init__. The 3.13 C warnings.warn(skip_file_prefixes=...) does not skip that frame (3.14 does), so warnings mis-attributed to <string> and the FastAPI-router attribution test failed under docker/3.13.

Replaced skip_file_prefixes with _subscriber_warn, which computes stacklevel by walking out to the first non-internal frame (this package / faststream / <string>) — robust across CPython versions and across the direct vs router paths (which differ in frame count).

Verification

• Existing validation tests are the regression guard, unchanged: test_unit.py reject/warn/no-warning cases (via @broker.subscriber), test_fastapi.py router-path attribution, test_fake.py direct-construction.
• just test: 543 passed, 100% coverage (docker / Python 3.13). just lint-ci: clean.

🤖 Generated with Claude Code