Fix LLM Proxy fractional reset times

[rohitjavvadi]

Summary

• Preserve fractional-second reset_time values from LLM Proxy quota stats.
• Keep the existing non-fractional ISO8601 fallback for plain timestamps.
• Add a regression test covering fractional reset timestamps.

Checks

• make format
• make lint
• swift test --filter LLMProxyUsageFetcherTests
• swift test
• swift build