docs: Add check_test_parity.sh usage across documentation

- Updated `AGENTS.md`, `README.md`, and `TESTING_GUIDELINES.md` with references
  and instructions for using the `scripts/check_test_parity.sh` script.
- Emphasized the importance of maintaining sync/async parity in tests.
- Included examples of the script's usage and default behavior for clarity.

Assisted-by: Codex

Author: datalogics-kam

AGENTS.md

@@ -19,6 +19,9 @@
 - `uv run pre-commit run --all-files` — enforce formatting and lint rules before
   pushing.
 - `uv run pytest` — execute the suite with the active interpreter.
+- `scripts/check_test_parity.sh` — run changed tests and report sync/async
+  parity gaps (accepts optional base/head refs, defaults to
+  `upstream/main..HEAD`).
 - `uv build` — produce wheels and sdists identical to the release workflow.
 - `uvx nox -s tests` — create matrix virtualenvs via nox and execute the pytest
   session.

README.md

@@ -36,3 +36,9 @@ Run the test suite with:
 ```bash
 uv run pytest
 ```
+
+Check sync/async parity for changed tests (defaults to `upstream/main..HEAD`):
+
+```bash
+scripts/check_test_parity.sh
+```

TESTING_GUIDELINES.md

@@ -13,6 +13,9 @@ iteration required.
   request customization, validation failures, file helpers, and live calls. Do
   not hide the transport behind a parameter; the test name itself should reveal
   which client is under test.
+- **Check parity regularly.** Run `scripts/check_test_parity.sh` (defaults to
+  `upstream/main..HEAD`) to spot missing sync/async counterparts, keeping
+  parameterized test IDs aligned between transports.
 - **Exercise both sides of the contract.** Hermetic tests (via
   `httpx.MockTransport`) validate serialization and local validation. Live
   suites prove the server behaves the same way, including invalid literal