test: add orientation getter and setter unit tests

[KazuCocoa]

Summary

Restore the intended orientation unit tests from #1289 following its revert in #1306. The existing orientation getter and setter already support the tested behavior.

Adds three tests in test/unit/webdriver/webdriver_test.py:

• Read LANDSCAPE from the orientation endpoint.
• Verify the request bodies when setting LANDSCAPE and PORTRAIT.
• Verify that an invalid orientation raises WebDriverException with the expected message.

The diff contains only these tests and their required imports (one file, 34 added lines). Assertions follow the current Ruff rules.

Validation

• Full test/unit/ suite passed using Python 3.14.6.
• All 24 tests in test/unit/webdriver/webdriver_test.py passed after the assertion style adjustment.
• Ruff lint and format checks passed for the changed file.
• git diff --check passed.

The test runs emitted httpretty deprecation warnings and thread warnings from the existing test_session_catches_error test.

[KazuCocoa]

Cc @Dor-bl