Skip linear mosaic targets in ImagingChunkedHandler

[thomaswilliamsastro]

Closes #350.

ImagingChunkedHandler crashed when given a linear mosaic target, since there's no MS to read. The standard ImagingHandler skips these gracefully.

• Check is_target_linmos near the top of __init__. For a mosaic, log a warning, set skip_target = True, chunk_params = {}, nchunks = 0, and return before anything touches the MS. Loops over range(nchunks) (as in the example script) then just do nothing.
• run_imaging and task_complete_gather_into_cubes return early with a warning when skip_target is set.
• New CASA-marked test tests/test_handlerImagingChunked.py, which builds minimal keys with a mosaic target and checks the handler skips it. It passes with the fix and fails on the old code (TypeError: cannot unpack non-iterable NoneType object during cell/imsize setup).

Note: a missing MS for a non-mosaic target still crashes; the standard handler skips that case too, but it's out of scope here.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 12.63%. Comparing base (24dcf46) to head (cf2841a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #413      +/-   ##
==========================================
+ Coverage    7.95%   12.63%   +4.67%     
==========================================
  Files          42       43       +1     
  Lines       16635    16700      +65     
  Branches     3983     3987       +4     
==========================================
+ Hits         1324     2110     +786     
+ Misses      15288    14356     -932     
- Partials       23      234     +211     

Components	Coverage Δ
casa	4.94% <ø> (ø)
handler	14.58% <75.00%> (+8.28%)	⬆️
spectral-cube	10.14% <ø> (ø)
utils	20.38% <ø> (+6.79%)	⬆️