Add generic try_import helper and adopt it for optional deps

[ghostiee-11]

Adds a single generic try_import(module_name, load=True) and adopts it across the optional-dependency helpers, so we do not grow a separate try_import_<dep> per PR. load=False returns the module only if it is already imported (a cheap hot-path check that never triggers an import); load=True imports on demand. The repeated geopandas test guard is also consolidated into a shared lumen/tests/utils.py.

[codecov]

Codecov Report

❌ Patch coverage is 87.75510% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.22%. Comparing base (59695a6) to head (ed0d1e4).

Files with missing lines	Patch %	Lines
lumen/tests/test_pipeline.py	50.00%	2 Missing ⚠️
lumen/sources/duckdb.py	50.00%	1 Missing ⚠️
lumen/tests/sources/test_optional_imports.py	92.30%	1 Missing ⚠️
lumen/tests/utils.py	87.50%	1 Missing ⚠️
lumen/util.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1926      +/-   ##
==========================================
- Coverage   71.22%   71.22%   -0.01%     
==========================================
  Files         198      199       +1     
  Lines       34645    34639       -6     
==========================================
- Hits        24677    24671       -6     
  Misses       9968     9968              

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