Resolve get_url paths from explicit repo root

[puneetdixit200]

Summary

• resolve relative data-index lookups from the repository root before using the DVC filesystem
• keep subrepo-aware key lookup anchored at the DVC filesystem root
• add regression coverage for dvc.api.get_url() when the current working directory is inside a subdirectory of the explicit repo path

Verification

• python -m pytest tests/func/api/test_data.py::test_get_url_with_explicit_repo_from_subdir -q (failed before implementation)
• python -m pytest tests/func/api/test_data.py::test_get_url_with_explicit_repo_from_subdir -q
• python -m pytest tests/func/api/test_data.py -k get_url -q
• PATH=/tmp/dvc-11029-venv/bin:$PATH python -m pytest tests/func/api/test_data.py -q
• python -m pytest tests/func/test_get_url.py -q
• python -m compileall -q dvc/repo/__init__.py tests/func/api/test_data.py
• python -m ruff check dvc/repo/__init__.py tests/func/api/test_data.py
• git diff --check

Fixes #11029.

AI-assisted: yes; I reviewed and tested the changes locally.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.99%. Comparing base (2431ec6) to head (900d5cc).
⚠️ Report is 203 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11042      +/-   ##
==========================================
+ Coverage   90.68%   90.99%   +0.30%     
==========================================
  Files         504      505       +1     
  Lines       39795    41153    +1358     
  Branches     3141     3263     +122     
==========================================
+ Hits        36087    37446    +1359     
- Misses       3042     3069      +27     
+ Partials      666      638      -28     

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