tests: add unit tests for dict_filter

[eeshsaxena]

Description

dvc.utils.dict_filter recursively drops a set of keys from a nested
dict/list structure (it backs dict_hash), but had no direct test coverage.

This adds tests/unit/utils/test_dict_filter.py covering:

• flat exclusion and multiple excluded keys
• nested keys being removed recursively
• an excluded key removing its whole subtree
• a list of dicts being filtered element-wise
• an empty exclude returning the input unchanged
• a scalar being passed through untouched

No source changes.

Verification

pytest tests/unit/utils/test_dict_filter.py - 7 passed. ruff check /
ruff format --check clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.98%. Comparing base (2431ec6) to head (c581f31).
⚠️ Report is 211 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11072      +/-   ##
==========================================
+ Coverage   90.68%   90.98%   +0.30%     
==========================================
  Files         504      506       +2     
  Lines       39795    41154    +1359     
  Branches     3141     3263     +122     
==========================================
+ Hits        36087    37444    +1357     
- Misses       3042     3071      +29     
+ Partials      666      639      -27     

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[eeshsaxena]

Hi! Gentle ping on this one; it's been open a couple of weeks and CI is green. Happy to rebase or make any changes if something's needed; whenever you get a chance to take a look, I'd really appreciate it. Thanks!

[skshetry]

Thanks for the patch, but I'd rather not add tests for such a simple utility. It has not been changed for a long time and is indirectly covered through dict_md5/dict_sha256 tests. Adding them now does not seem to add value. Sorry.