Fix broadcast list unwrapping and mixed positional list handling

[minghangli-uni]

This is a follow-up fixup PR to #88, which unintentionally corrupted some previously working behaviours.

1. single item list broadcasting produces incorrect results

Before this change, when a scalar parameter was provided as a single element yaml list - a common pattern for broadcasting across multiple branches, the existing generator preserved the list shape instead of unwrapping it to scalar, such as given,

    config.yaml:
      walltime:
        - 5:00:00

the generator output became,

walltime:
  - 5:00:00

This is not the intended semantics for scalar parameters. The current PR fixes this issue by treating a single element list as a broadcasted scalar and unwrapping it to,

walltime: 5:00:00

2. Nested yaml list-of-lists handling during list merges

The existing list-merge logic treated an outer list wrapper as a single positional element in the base list, such as,

input:
  - - a.nc
    - b.nc

was interpreted as replacing the first element of the base list input[0] with the list [a, b], resulting in,

input:
  - [a.nc, b.nc]
  - ocean_hgrid.nc
  - ocean_mosaic.nc
 ...

This PR fixes the issue by unwrapping the broadcast wrapper from [[a, b]] to [a, b] before merging or cleaning, so the above update now correctly produces,

input:
  - a.nc
  - b.nc
  - ocean_mosaic.nc
...

3. Support mixed positional inventories with PRESERVE / REMOVE markers

For this special case which mixes positional inventories, such as below the inner list does not include all maps inside but can be a special marker (REMOVE / PRESERVE), it does not get per-branch selection correctly.

submodels:
  - - PRESERVE
    - input:
        - - a.nc
          - b.nc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.98%. Comparing base (3c2558e) to head (bb95ba4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   98.96%   98.98%   +0.01%     
==========================================
  Files          15       15              
  Lines         679      692      +13     
  Branches      160      165       +5     
==========================================
+ Hits          672      685      +13     
  Misses          3        3              
  Partials        4        4              

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