When a CI run hits a failing assertResponseStatusCodeSame() (or any of the verbose-by-default BrowserKit assertions), the failure message includes the full response body. For an HTML page that's hundreds to thousands of lines per failing test, which makes the CI log unreadable when a regression touches many controllers — the original report counts 2276 lines for one test.

Two existing escape hatches:

• per-call $verbose: false — boring to repeat;
• self::setBrowserKitAssertionsAsVerbose(false) in a PHPUnit bootstrap — requires writing/registering a bootstrap file just for this knob.

This PR makes the default also honor the SYMFONY_BROWSERKIT_ASSERTIONS_VERBOSE env var (filter_var boolean parsing), so the same toggle can live in phpunit.xml:

<phpunit>
    <php>
        <env name="SYMFONY_BROWSERKIT_ASSERTIONS_VERBOSE" value="0"/>
    </php>
</phpunit>

setBrowserKitAssertionsAsVerbose() still wins over the env var, and the per-call $verbose argument still wins over both. Default with neither set stays true — fully BC.

Test plan:

• BrowserKitAssertionsTraitTest::testFailureMessageIsVerboseByDefault — BC guard: with no env var and no setter call, the response body is included in the failure message.
• BrowserKitAssertionsTraitTest::testEnvVarOptsOutOfVerboseFailureMessage — with the env var set to 0, the response body is not included.
• BrowserKitAssertionsTraitTest::testExplicitSetterWinsOverEnvVar — precedence guard: explicit setBrowserKitAssertionsAsVerbose(true) overrides the env var.

Each test runs in a separate process so the trait's static $defaultVerboseMode starts fresh. RED check passes: reverting the helper makes testEnvVarOptsOutOfVerboseFailureMessage fail (response body still included). GREEN: all three pass (3 tests, 6 assertions).

RED/GREEN verification

Without the fix (BrowserKitAssertionsTrait.php reverted to base on upstream/8.1):

1) testEnvVarOptsOutOfVerboseFailureMessage
Failed asserting that 'Failed asserting that the Response status code is 500.
HTTP/1.0 200 OK
Cache-Control: no-cache, private
... [full response dump] ...'
does not match expected condition (non-verbose).

FAILURES!
Tests: 3, Assertions: 6, Failures: 1, PHPUnit Warnings: 2.

With the fix:

Tests: 3, Assertions: 6, PHPUnit Warnings: 2.

testEnvVarOptsOutOfVerboseFailureMessage is the test that targets the regression itself (env-var resolution path); it fails on the base branch and passes once the patch is restored. The other two new tests are backwards-compat guards (testFailureMessageIsVerboseByDefault for default behavior, testExplicitSetterWinsOverEnvVar for setter precedence) — they pass on both sides by design, locking in the surrounding contract.