Fix stats days filter for missing and invalid timestamps

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53249.

Summary

• Require sessions to have a valid RFC3339 timestamp before they can be included by stats --days.
• Normalize parsed session timestamps to UTC before comparing with the rolling start date.
• Add regression coverage for missing and invalid timestamps.

Validation

• cargo +1.90.0 fmt --package cortex-cli --check
• git diff --check
• Isolated rustc --test harness for date-range filtering behavior

Note: targeted cargo test -p cortex-cli ... still fails before executing this test on this Windows/MSVC environment due an existing aws-lc-sys C build failure.

[greptile-apps]

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