Assertion capture

[vishnuv688]

Part of #277

No assertion rows are captured. The JS side patches node:assert
through core/assert-patcher and renders passing and failing rows; a pytest
assertion is a rewritten assert statement, so the mechanism has to differ.

Acceptance criteria

Failing and passing assertions appear as rows carrying
expected and actual. The Errors tab collects the failures.

[vishnuv688]

Scope note: plain scripts have no interception point

This issue is written around pytest, whose assertion rewriting is what makes interception possible. A plain script has no equivalent hook: assert is a bare language statement, and the adapter instruments WebDriver.execute, so an assert never touches the driver and cannot be observed at all.

Reproduced on the new login example (examples/selenium/python-test/login.py) with a deliberately broken assertion. Neither passing nor failing assertions produce a row, and nothing in the current acceptance criteria would change that for this style of run.

So the acceptance criteria need to say which of these is in scope:

• pytest — hook the rewritten assertion (pytest_assertrepr_compare or the plugin's own reporting) and attach rows to the running test. This is what the issue assumes today.
• plain script — there is no interception point, so it needs either an opt-in API the user calls (e.g. devtools.expect(...)) or an explicit statement that plain scripts are out of scope.

Worth resolving before implementation, because the two need different mechanisms and the plain-script path is the one the example in the docs uses.

Related but distinct, and currently untracked

The same run surfaced a separate defect that this issue would not fix: _send_default_suite is called with a hardcoded "passed" (packages/selenium-devtools-py/src/selenium_devtools/instrumentation.py:356) and the adapter has no failure path, so a plain-script run that raises still reports a green 1/1. The command stream stops at the failing line while the test tree claims a pass. Independent of assertion capture and needs its own issue.