fix: clamp negative Retry-After delays before sleeping

[abhinavkr26104]

Summary

• clamp parsed Retry-After and Retry-After-Ms delays to Duration.ZERO when they are negative
• preserve immediate retries for past HTTP-date values
• add sync and async regression coverage for negative numeric delays and past dates

Fixes #853

Testing

• git diff --check passed
• direct ktfmt 0.61 validation passed for both changed Kotlin files
• full Gradle test suite was attempted online and offline; the online run timed out during Kotlin/Gradle execution without diagnostics, while the offline run was blocked by uncached junit-jupiter-engine:5.9.3
• repository lint tasks were attempted; direct ktfmt passed, while Gradle lint was blocked by the same local dependency/compile environment