docs: Refine parallelism settings and testing customization guidelines

- Updated `AGENTS.md` with `pytest -n auto` to simplify parallel test configuration.
- Enhanced guidance for endpoint/server option testing:
  - Unit tests for `extra_query`, `extra_headers`, `extra_body`, and `timeout`.
  - Live tests only required for server-observable effects.
- Expanded `TESTING_GUIDELINES.md` to clarify customization validation.
  - Mocked tests handle request customization.
  - Live tests validate server-specific behaviors when applicable.

Assisted-by: Codex

Author: datalogics-kam

AGENTS.md

@@ -23,7 +23,7 @@
 - `uvx nox -s tests` — create matrix virtualenvs via nox and execute the pytest
   session.
 - `nox` executes pytest sessions with built-in parallelism; when invoking pytest
-  directly use `pytest -n 8 --maxschedchunk 2` to mirror the parallel test
+  directly use `pytest -n auto --maxschedchunk 2` to mirror the parallel test
   scheduling and keep runtimes predictable.
 - Coverage reports (XML/Markdown/HTML) are produced by the nox `tests` session
   and stored under `coverage/py<version>/` (for example,
@@ -134,7 +134,9 @@
   considered complete. Mirror the naming/structure used by the graphic
   conversion suites: one module per endpoint, parameterized success cases that
   enumerate all accepted literals, at least one invalid input that hits the
-  server, and coverage for any request options surfaced on the client. If an
+  server, and coverage for server-observable endpoint options. Validate
+  `extra_query`/`extra_headers`/`extra_body`/`timeout` plumbing in unit tests
+  (MockTransport) unless a live assertion depends on those options. If an
   endpoint cannot be exercised live, call that out explicitly in the PR
   description with the reason and the follow-up plan; otherwise reviewers should
   block the change. Treat this as a release gate on par with unit tests.

TESTING_GUIDELINES.md

@@ -201,15 +201,22 @@ iteration required.
   predictable. Use `pytest.fixture(scope="module"/"class")` and
   `pytest_asyncio.fixture` to cache uploaded PDFs/profiles for both transports.
 - **Sync + async parity:** Every live module should contain matching sync and
-  async tests for success, customization, streaming, and invalid paths
-  (compression levels, conversion options, file streaming helpers).
+  async tests for success, streaming, and invalid paths (compression levels,
+  conversion options, file streaming helpers). Treat transport-plumbing
+  customization (`extra_query`, `extra_headers`, `extra_body`, `timeout`) as a
+  unit-test responsibility unless the endpoint exposes server-observable
+  behavior tied to those options.
 - **Enumerate literals:** Parameterize over every accepted literal (compression
   levels, `color_model`, `smoothing`, merge selectors, redaction presets). Each
   literal should hit the server once per transport.
 - **Optional arguments:** Exercise options such as custom output prefixes,
   diagnostics toggles, merge metadata, and URL uploads. Validate the server
   honors them (filenames start with the user-provided prefix, warnings appear
   when expected).
+- **Customization placement:** Verify request customization wiring in mocked
+  tests (sync + async) for each endpoint. Live customization tests are optional
+  and should be added only when they validate server-side behavior beyond client
+  request assembly.
 - **Negative live cases:** Override JSON via `extra_body`/`extra_query` to
   bypass local validation and assert `PdfRestApiError` (or the exact server
   exception) surfaces—for example, sending an invalid compression literal or