Fix holiday fetching URL and add ArgentinaDatos provider

[PPeitsch]

Description

This PR fixes the holiday fetching logic which was failing for 2026/2027 due to a URL change in argentina.gob.ar.
Additionally, it implements a new ArgentinaApiProvider that fetches data from the ArgentinaDatos API (api.argentinadatos.com) as a robust alternative to HTML scraping.

Type of Change

• Bug fix
• New feature
• Refactoring
• Documentation update
• UI/UX improvement
• Code style update
• Test coverage
• Configuration change

Testing

• Unit Tests: Ran pytest tests/test_services.py and tests/test_argentina_api_provider.py to verify both the legacy provider (mocked) and the new API provider.
• Type Checking: Verified code compliance with mypy.
• Formatting: Applied black and isort.

Checklist

• I have followed the project's code style (Black, isort, PEP 8)
• My code generates no new warnings
• I have added tests for new functionality
• All tests pass locally
• I have updated the documentation where necessary
• I have run pre-commit hooks before submitting

Fixes #43

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

Files with missing lines	Coverage Δ
app/config/config.py	100.00% <100.00%> (ø)
...rvices/holiday_providers/argentina_api_provider.py	100.00% <100.00%> (ø)
app/services/holiday_service.py	100.00% <100.00%> (ø)

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