Fix NaN handling for TimeIntervals bounds

[AtomicGlance]

Fixes #2212

What changed

• Ignore NaN start times in TimeIntervals.get_starting_time.
• Calculate duration from the earliest valid start to the latest known start or stop boundary, so an ongoing interval with a missing stop does not shorten the observed span.
• Return None for an empty table and NaN when no valid start time exists.
• Add regression coverage for mixed and all-NaN inputs and document the behavior in the upcoming changelog.

Testing

• python -m pytest tests/unit/test_epoch.py -q — 26 passed
• ruff check src/pynwb/epoch.py tests/unit/test_epoch.py — passed
• The broader unit suite reports 594 passed and 2 skipped; its only failures are the existing typemap-cache tests, which require the missing NWB schema submodule in this partial checkout.

[AtomicGlance]

I updated this branch onto the current PyNWB dev branch and resolved the release changelog conflict. All 26 tests in ests/unit/test_epoch.py pass locally, including NaN starts/stops and ongoing-interval cases. The diff remains limited to TimeIntervals behavior, tests, and the changelog. It is ready for review when convenient.

[AtomicGlance]

I rechecked the refreshed branch against the local source tree: the NaN/ongoing interval cases pass (5 passed), including all-NaN and later-start boundaries. No new code changes were needed; the branch is current and ready for review.