test(adapters/linear): assert the deadline branch fires in TestRateLimitDeadlineReturnsTypedRateLimited #111

Follow-up from #110 (an advisor suggestion, not a bug).

TestRateLimitDeadlineReturnsTypedRateLimited (adapters/linear/harden_capability_test.go) checks that a 429 whose backoff would end past the caller's deadline returns *linear.RateLimited. It does not check which branch produced that error. If a later retry-policy change sent the error out through a different check, the test would still pass.

Acceptance: the test also asserts that the deadline branch in adapters/linear/retry.go fired (for example, the "deadline" log line or an equivalent observable), and it fails when that branch is disabled. Keep the test free of timing assumptions, as #110 made it.

This gap existed before #110.

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