Fixes time gap rendering in pcolormesh spectrogram plots

[MohamedAli1937]

PR Description

Resolves the TODO in radiospectra/spectrogram/sources/eovsa.py that noted time gaps in spectrogram plots need to be rendered as actual gaps instead of being stretched across by pcolormesh.

fixes: #180

Solution:

• Adds a _insert_time_gaps() helper to PcolormeshPlotMixin that detects gaps in the time axis and inserts NaN rows + midpoint timestamps so pcolormesh renders them as empty space.

Changes:

radiospectra/mixins.py:

• Adds _insert_time_gaps(times, data, gap_threshold=None) static method.
• Updates plot() with handle_gaps=True and gap_threshold=None parameters. The feature is enabled by default and backward compatible.

radiospectra/spectrogram/sources/eovsa.py:

• Removes the outdated TODO comment.

radiospectra/tests/test_mixins.py (new):

• Adds 5 unit tests covering no-gap, single-gap, multiple-gap, float-input, and manual-threshold cases.

AI Assistance Disclosure

AI tools were used for:

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding
• No AI tools were used

Regardless of AI use, the human contributor remains fully responsible for correctness, design choices, licensing compatibility, and long-term maintainability.

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