add comprehensive docstrings to test fixtures

[rohan-019]

Summary

This PR improves test maintainability and onboarding by adding clear, Google-style docstrings to all fixtures in tests/conftest.py and by hardening a few fixtures to avoid brittle behavior and cross-test side effects. No production code behavior is changed — only test utilities and their documentation.

Related issue #137

What I changed

• Added concise, Google-style docstrings to every @pytest.fixture in tests/conftest.py, including:
  • One-line summary
  • Detailed description (when helpful)
  • Returns: or Yields: section
  • Small Example: blocks for complex fixtures
• Removed duplicate/stray docstrings and consolidated docstrings so each fixture has a single docstring.
• Made file loading robust by switching relative file reads to Path(__file__).parent / "data" / ....
• Prevented accidental cross-test mutation:
  • Use copy.deepcopy in test_allergy_with_reaction and test_medication_with_dosage to avoid mutating fixtures passed as inputs.
• Fixed doc_ref_without_content to construct DocumentReferenceContent + Attachment (instead of a plain dict) for FHIR object consistency.
• Ensured config_fixtures yields a temporary config directory and writes representative YAML files used by config-related tests.
• Removed duplicate real_config_dir definitions (kept the robust implementation that pytest.skips when configs are absent).
• Added a usage example to cda_adapter docstring.
• Small type / wording clarifications in docstrings.

Why

• New contributors (and reviewers) struggled to understand what each fixture provides and when to reuse it.
• Several fixtures were fragile:
  • Relative paths could fail depending on the working directory.
  • Fixtures that mutated objects handed in by other fixtures could create subtle test ordering bugs.
  • Inconsistent construction of FHIR resources made tests harder to reason about.
• Improving docstrings + robustness reduces onboarding friction and makes tests safer to edit.

Acceptance criteria

• ✅ All fixtures now have a docstring with at least a one-line summary, description and Returns/Yields.
• ✅ Docstrings follow a consistent Google-style structure (one-line summary, body, Returns/Yields, Example where useful).
• ✅ Related fixtures are referenced in See Also or examples where appropriate.
• ✅ No behavioral change to production code. Tests are hardened to avoid cross-test mutation and path issues.

[rohan-019]

Hi, @jenniferjiangkells Please review!!

[jenniferjiangkells]

lgtm