Fix rate limit reset duration parsing

[SatorSK]

Fixes PlatformNetwork/bounty-challenge#52868.

Summary

• Parse rate-limit reset headers as duration strings such as 1s, 1m30s, and 6m0s
• Treat small integer reset values as seconds from now while preserving large Unix timestamp values
• Add a reset-token fallback and focused unit coverage

Why

parse_rate_limit_headers() previously parsed x-ratelimit-reset-requests only as a Unix timestamp. Providers can return reset values as durations, for example OpenAI-style 60s / 1m30s or plain integer seconds. Those values were silently ignored, leaving reset_at unset and preventing display/retry code from knowing when the window resets.

Verification

• cargo fmt --package cortex-ratelimits --check
• cargo test -p cortex-ratelimits

[greptile-apps]

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