fix(gateway): classify Photon target_not_allowed as a dead target

[pierrenode]

Summary

PhotonAdapter._sidecar_send() preserves str(e) which includes target_not_allowed text when a shared/free-tier Photon line permanently cannot open a new outbound thread to a target. classify_send_error() didn't recognize this shape, so mark_dead() was never called and the target retried forever on every send.

classify_send_error() now recognizes target_not_allowed as a "forbidden"-class dead target — matching the existing docstring, which already describes this bucket as "lacks permission to post to the target".

Scope note (updated)

An earlier revision of this PR also carried scheduler-level wiring (cron/scheduler.py::_deliver_result() consulting/updating its own DeadTargetRegistry around the live-adapter and standalone paths, since production cron delivery calls DeliveryRouter._deliver_to_platform() directly and never went through DeliveryRouter.deliver()'s dead-target check). That wiring has been removed from this PR: it duplicates — with a narrower implementation (no media-only-job coverage, no shared registry passed into DeliveryRouter) — the fix already carried by #64915, which is intentionally the PR that owns that call site. Landing both would conflict on the same lines with divergent implementations.

This PR is back to its original, narrow scope: only the classify_send_error() classification fix.

Test plan

• tests/gateway/test_dead_targets.py::test_photon_target_not_allowed_marks_target_dead — DeliveryRouter.deliver()-level test proving the classifier recognizes the Photon error shape end-to-end (first delivery marks dead, second is skipped).
• tests/gateway/test_send_error_classification.py — unit coverage for the new target_not_allowed → "forbidden" classification.
• Mutation-verified: reverting the gateway/platforms/base.py change breaks both of the above.
• Ran the full neighboring suite: tests/gateway/test_dead_targets.py + tests/gateway/test_send_error_classification.py + tests/gateway/test_send_multiple_images.py (37 passed) — all green.
• ruff check clean on all changed files.
• Rebased onto current upstream/main, single commit.

[teknium1]

Thanks for tracing the Photon error classification path. The classifier gap is real: PhotonAdapter._sidecar_send() preserves str(e) (plugins/platforms/photon/adapter.py:2473-2482), and that string includes target_not_allowed (plugins/platforms/photon/adapter.py:262-282).

Problems

• The claimed cron fix does not reach production cron delivery. cron/scheduler.py:1789-1806 calls router._deliver_to_platform(...) directly. Dead-target lookup and mark_dead() only occur in DeliveryRouter.deliver() (gateway/delivery.py:341-385), so the new branch in gateway/platforms/base.py cannot short-circuit a later cron tick.
• The added regression test calls router.deliver() (tests/gateway/test_dead_targets.py:79-85), covering the wrapper that cron bypasses rather than the reported scheduler path.

Suggested changes

• Route the scheduler's live and fallback delivery paths through a dead-target-aware public API, while preserving its existing topic-routing and fallback semantics.
• Add a scheduler-level regression covering two Photon cron deliveries and asserting the second does not invoke the live adapter or standalone fallback.

Automated hermes-sweeper review.

[teknium1]

Blocking: cron does not call the wrapper that consumes this classification. cron/scheduler.py:1789-1806 invokes _deliver_to_platform() directly, while mark_dead() is only reached from DeliveryRouter.deliver() at gateway/delivery.py:376-385; this match therefore will not short-circuit a later cron tick.