chore: improve test guidelines in AGENTS.md

[nabinchha]

Summary

• Add explicit test guidelines: test public APIs only (no _-prefixed internals), require type annotations on tests/fixtures, keep imports at module level
• Fix fixture reference to reflect the actual pytest_plugins layout (data_designer.config.testing.fixtures / data_designer.engine.testing.fixtures) instead of a nonexistent tests/conftest.py
• Recommend ids= with @pytest.mark.parametrize for readable test names

Test plan

• Verify AGENTS.md renders correctly on GitHub

Made with Cursor

[greptile-apps]

Greptile Summary

This PR improves the test guidelines section of AGENTS.md with three targeted documentation fixes:

• Fixture reference corrected: The old bullet pointed to a non-existent top-level tests/conftest.py. The new text accurately reflects the actual pytest_plugins declarations found in every package conftest.py — data_designer.config.testing.fixtures and data_designer.engine.testing.fixtures — which was confirmed to match the real fixture files on disk.
• New test guidelines added: Three new rules — test public APIs only, require type annotations on tests/fixtures, and keep imports at module level — are consistent with existing project conventions (isort, ruff annotations rules) and the rest of the guide.
• ids= recommendation: Adds a small but useful clarification to the @pytest.mark.parametrize guideline for readable test names.
• Blank line fix in code example: Adds the missing blank line between the import block and the function definition, conforming to PEP 8 / ruff E302.

Confidence Score: 5/5

• This PR is safe to merge — it is a pure documentation change with no code logic or behaviour modifications.
• All changes are limited to AGENTS.md. The fixture module paths added (data_designer.config.testing.fixtures, data_designer.engine.testing.fixtures) were verified to match the actual files on disk and the pytest_plugins declarations in the real conftest.py files. The new guidelines are accurate, self-consistent, and aligned with the existing project conventions. No functional code is touched.
• No files require special attention.

Important Files Changed

Filename	Overview
AGENTS.md	Documentation-only update: fixes the fixture reference from a non-existent tests/conftest.py to the correct pytest_plugins module paths, and adds three new, accurate test guidelines (public-API-only, type annotations, module-level imports) plus a ids= recommendation for parametrize.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[pytest test run] --> B{Package conftest.py}
    B --> C[pytest_plugins declaration]
    C --> D[data_designer.config.testing.fixtures]
    C --> E[data_designer.engine.testing.fixtures]
    D --> F[stub_data_designer_config_str\nand other config fixtures]
    E --> G[engine-level shared fixtures]
    B --> H[Local conftest.py\nin each test directory]
    H --> I[Package/module-specific fixtures]
    F & G & I --> J[Individual test functions]

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