Fix pre-epoch fractional SystemTime conversion

[fallintoplace]

Summary

• fix EdmDateTimeOffset to SystemTime conversion for fractional timestamps before the Unix epoch
• borrow from the seconds field before subtracting negative fractional timestamps from UNIX_EPOCH
• add a regression test for 1969-12-31T23:59:59.5Z

Validation

• cargo fmt -p nv-redfish-core -- --check
• cargo test -p nv-redfish-core

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.