fix: match boolean environment values exactly

[lprnmns]

Problem

env2bool uses an unanchored regex search, so unrelated values containing y or 1, such as my_branch and v1.0, enable DVC environment flags. Only complete recognized affirmative tokens should evaluate to true.

Fixes #11080.

Fix

Normalize surrounding whitespace and case, then compare against the existing affirmative vocabulary: 1, y, yes, and true. Unset-variable behavior and recognized values are unchanged.

Tests

• uv run pytest tests/unit/utils/test_utils.py::test_env2bool_matches_the_entire_value -q — 9 passed
• uv run pytest tests/unit/utils/test_utils.py -q — 43 passed, 1 skipped (Windows-specific)
• uvx ruff check dvc/utils/__init__.py tests/unit/utils/test_utils.py — passed
• uvx ruff format --check dvc/utils/__init__.py tests/unit/utils/test_utils.py — passed
• git diff --check — passed

• ❗ I have followed the Contributing to DVC checklist.

• 📖 This focused behavior fix does not require a documentation update.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.98%. Comparing base (2431ec6) to head (cf8d009).
⚠️ Report is 213 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11093      +/-   ##
==========================================
+ Coverage   90.68%   90.98%   +0.30%     
==========================================
  Files         504      505       +1     
  Lines       39795    41142    +1347     
  Branches     3141     3263     +122     
==========================================
+ Hits        36087    37432    +1345     
- Misses       3042     3071      +29     
+ Partials      666      639      -27     

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[lprnmns]

Closing this because the same env2bool exact-match fix is already being worked on in #11081 and #11082. Sorry for the duplicate.