Move the schema-validation helpers out of tests/conftest.py

`tests/conftest.py` (349 lines) holds two unrelated things: 10 pytest fixtures, and 5 plain helper functions: `validate_covjson`, `assert_schema_valid`, `parse`, `roundtrip`, `roundtrip_is_stable`.

The fixtures are fine: pytest auto-discovers them and nothing imports them. The helpers get no auto-discovery, so every test module that wants one writes `from conftest import ...`, which pytest's own docs advise against:

> If you have `conftest.py` files which do not reside in a python package directory (i.e., one containing an `__init__.py`) then "import conftest" can be ambiguous... It is thus good practice for projects to either put `conftest.py` under a package scope or to never import anything from a `conftest.py` file.
> — https://docs.pytest.org/en/stable/how-to/writing_plugins.html

We satisfy neither branch: there is no `tests/__init__.py`, and 6 modules import from conftest (`test_conftest`, `test_factory`, `test_helpers`, `test_modeler`, `test_spec_roundtrip`, `test_playground_roundtrip`).

**Severity: latent, not live.** The hazard the docs name is ambiguity between multiple `conftest.py` files on `sys.path`, and this repo has exactly one, so nothing is broken today. The guidance is "good practice", not a MUST. It becomes a problem the day a second conftest appears (e.g., `tests/integration/conftest.py`).

**Suggested fix:** move the 5 helpers to a normal test-support module and leave conftest to its fixtures. That takes the docs' "never import anything from a conftest.py" branch and also fixes the underlying cohesion problem, which is the real one: a 349-line conftest doing two jobs. Adding `tests/__init__.py` is the other sanctioned branch, but it only legitimizes the import and leaves the two jobs fused.

The module name is currently undecided. Note `tests/helpers.py` would be actively confusing next to `tests/test_helpers.py`, which tests `src/titiler_covjson/helpers.py`. Candidates: `tests/support.py`, `tests/covjson.py`.

Touches the 6 importing modules and the AGENTS.md testing-conventions section (L173), which currently codifies the pattern: "`tests/conftest.py` provides `validate_covjson` / `assert_schema_valid`".

Start with tests/conftest.py and the six importing modules: test_conftest, test_factory, test_helpers, test_modeler, test_spec_roundtrip, and test_playground_roundtrip. Choose a normal test-support module, move the five named helpers there, update imports and the AGENTS.md testing-conventions section, then run the test suite to confirm all tests pass.

Written by the indexing model from the issue text.