This commit integrates Test262 test execution into the WPT Taskcluster
Continuous Integration system.

Key changes and their purpose:
- **Task Definition (`tools/ci/tc/tasks/test.yml`):**
  - A new `wpt-test262` component is defined, specifying the common
    properties and chunking strategy for Test262 tasks within Taskcluster.
  - The main Taskcluster task generation matrix is updated to include
    `test262` as a recognized test suite. This ensures that Taskcluster
    will automatically create jobs for Test262 across configured
    browsers and channels (e.g., Firefox Nightly, Chrome Canary).
- **CI Validation (`tools/ci/tc/tests/test_valid.py`):**
  - The integration tests for Taskcluster's decision logic (`test_valid.py`)
    are updated to expect the new `wpt-*-test262-*` tasks in the generated
    task graph. This provides automated validation that the CI configuration
    is correctly interpreted and will schedule the Test262 tasks as intended.

With these changes, Taskcluster will automatically detect and execute Test262
tests that are present in the `third_party/test262` directory (as populated by
the automated vendoring workflow).

This work is a fundamental part of the broader Test262 integration effort,
as detailed in the RFC: web-platform-tests/rfcs#229

This commit is the final in a series of smaller PRs split from the larger,
original implementation in #55997.