Issue #3586299: Fix test and documentation consistency before rc1.

Description

Addresses sub-items 7.1, 7.2, and 7.4 from #3586299 (closed).

7.1 — Integration test process isolation

Standardized process isolation on all remaining Functional and FunctionalJavascript test classes using the #[RunTestsInSeparateProcesses] PHPUnit attribute, matching #3586280 (closed) and Drupal 11.3+ requirements.

• Converted 18 Functional / FunctionalJavascript classes from PHPDoc @runTestsInSeparateProcesses to the attribute
• Updated conventions in tests/README.md and docs/contribute/testing.md

7.2 — Replace documentation-only tests

AiContextScopeGlobalTest

Replaced five placeholder assertions (assertTrue(TRUE, '...')) with real unit-level assertions:

• supportsSubscriptions()
• allowsMultiple()
• isDynamic()
• getCurrentValue()
• buildValueForm() with $for_subscription = TRUE

AiContextItemTranslationHandlerTest

Removed this test class. It only verified PHP language features (is_subclass_of() and method_exists()) rather than module behavior. Translation handler behavior is already covered by existing Functional and Kernel tests.

7.4 — Test environment documentation

Updated tests/README.md with SIMPLETEST_BASE_URL and SIMPLETEST_DB guidance for DDEV, aligned with values in docs/contribute/testing.md (http://localhost, mysql://db:db@db:3306/db). Also links to the full setup doc for phpunit.xml and browser output directory configuration.

Moved to follow-up issues

• 7.3 — Functional test dual-container traps → #3586325
• 7.5 — Accessibility test coverage → #3586326

Test plan

• ./lint.sh passes (phpcs, phpstan, cspell, eslint, stylelint)
• CI phpunit pipeline green
• No production code changed; test and documentation changes only

Post-merge steps

None required.

Checklist

• I have updated the MR title to use format: Issue #1234: My issue title.
• I have updated the MR description to include: Closes #1234
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

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Closes #3586299 (closed)