Harden branded-variable disambiguations

[Copilot]

Describe your changes

Port of noaa-gfdl/fre-cli#832. Brand extraction used split('_') indexing, which breaks when target_var contains underscores and raises IndexError on non-branded MIP entries. Error messages on disambiguation failure were either empty (bare ValueError) or lacked diagnostic context.

Brand extraction (cmor_mixer.py):

• Replace mip_var.split('_')[0] / [1] with startswith(target_var + '_') + prefix stripping
• Guard against empty brand suffix (e.g. trailing _)
• Bare ValueError → message with target var, dim count, available branded entries

# Before — breaks on target_var="sea_surface_temp"
target_var == mip_var.split('_')[0]        # "sea" != "sea_surface_temp"
brands.append(mip_var.split('_')[1])       # IndexError on "sos"

# After
brand_prefix = target_var + '_'
if mip_var.startswith(brand_prefix):
    brand = mip_var[len(brand_prefix):]

Disambiguation errors (cmor_helpers.py):

• Add empty-brands-list guard at filter_brands() entry
• "All eliminated" and "multiple remain" errors now include has_time_bnds, input_vert_dim, candidate brands, and actionable guidance

Tests (test_cmor_helpers.py):

• test_filter_brands_empty_brands_list
• test_filter_brands_all_eliminated_message_contains_properties
• test_filter_brands_multiple_remain_message_contains_properties

Issue ticket number and link

Checklist before requesting a review

• I ran this code and know what it does
• I commented the code and think it is readable
• I wrote new tests to cover newly added lines
• I wrote new documentation to explain new functionality
• pylint and pytest checks are passing
• No print statements; all user-facing info uses logging module

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
fremorizer/cmor_mixer.py	78.57%	3 Missing ⚠️

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
- Coverage   95.19%   95.14%   -0.05%     
==========================================
  Files          10       10              
  Lines        1310     1318       +8     
==========================================
+ Hits         1247     1254       +7     
- Misses         63       64       +1     

Files with missing lines	Coverage Δ
fremorizer/cmor_helpers.py	94.28% <100.00%> (+0.03%)	⬆️
fremorizer/cmor_mixer.py	91.93% <78.57%> (-0.10%)	⬇️

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