test(route): cover 429 tool continuation after free failover

[seonghobae]

Scope

Extend #1279's real HTTP, two-request orchestrator/free tool-session check to a provider-declared 429 with finite retry timing. The first request must advance to an eligible free worker, return its tool call and typed attempt receipt, and accept the caller's tool result on the next request through the serving worker.

This is one parameterized acceptance case over the existing 502 fixture. It changes no production routing policy or retry budget.

Verification

• Exact branch head 962c4144af480e6013b486e9c5630b3571409fdf.
• Python 3.12: tests/test_actions_model_fallback.py — 34 passed, process exit 0.
• Ruff E4/E7/E9/F and git diff --check passed.
• The case is local HTTP contract evidence, not a deployed 2,100-second session proof.

Merge boundary

Draft and stacked on #1279. Keep #1279's queued Security run untouched. After the parent reaches protected main, retarget and require this PR's own exact-head hosted checks and independent review before merging.

Refs #1016.

Current review admission — 2026-09-28

This head remains 962c4144af480e6013b486e9c5630b3571409fdf on exact parent #1279 42d78c55a0e58df8f033a8b58616c2f77897ca8b. Its recorded Python 3.12 focused suite passed 34 tests on this unchanged head. The PR is now Ready so its own non-skipped hosted checks and independent review can run. Parent #1279 is still open with queued checks; local evidence does not approve either PR. The earlier Draft statement is historical.

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[seonghobae]

Exact-head admission correction — 58dec5c3c0bcb8aca79190109f3fa55aad4b08d1

Ready is review admission only. Fresh audit against base a115ec2fe33f4be80a7f4a189d26d9a000d23636 found:

• latest terminal workflow blockers: Security and Quality 36378068296=cancelled

This PR is moved to Draft/Proposed until the causal owner repair is present on a successor exact head and re-audited. Queued/pending work is neither an additional blocker nor passing evidence. No Close, force push, destructive rebase, manual rerun, synthetic status/approval, merge, auto-merge, or bypass was performed.