[Alan4506]

Description of changes:

The announcement says when AWS_NEW_RETRIES_2026=true is set, DynamoDB and DynamoDB Streams use 11 attempts in legacy mode and 4 in standard mode. However, when max_attempts is not set explicitly, both clients get the wrong counts:

AWS_NEW_RETRIES_2026=true, no max_attempts	Observed	Expected (#3285)
DynamoDB, AWS_RETRY_MODE=legacy	3	11
DynamoDB, AWS_RETRY_MODE=standard / adaptive	3	4
DynamoDB, 'retries' => ['mode' => 'standard'/'adaptive']	3	4
DynamoDB Streams, AWS_RETRY_MODE=legacy	12	11
DynamoDB Streams, AWS_RETRY_MODE unset or =standard	12	4

Three causes, one fix each:

1. Mode from env / shared config. DynamoDbClient::_defaultRetries() reads the mode through ConfigurationProvider::env() and ini(). When max_attempts isn't set, those providers fill in the generic 3, and _defaultRetries() can't tell that 3 apart from a user-supplied 3, so it never applies the DynamoDB value.
   Fix: after the providers resolve, _defaultRetries() checks AWS_MAX_ATTEMPTS / max_attempts directly via ConfigurationResolver::resolve(). If neither is set, it replaces the attempt count with 11 (legacy) or 4 (standard / adaptive) for the resolved mode.

2. Mode from the client retries array. DynamoDbClient::_applyRetryConfig() gets its config from ConfigurationProvider::unwrap(), which fills in 3 for an array without max_attempts. The legacy branch already corrects this to 11 in resolveLegacyModeMaxRetries(); the standard / adaptive branch didn't.
   Fix: a sibling resolveStandardModeConfig() applies the same check (flag on, array config, no max_attempts) and returns a config with 4.

3. DynamoDB Streams ignores the flag. DynamoDbStreamsClient hardcodes retries.default = 11 with no opt-in check, so a Streams client with no retries option always builds the legacy middleware and never reads env / shared config.
   Fix: DynamoDbStreamsClient now picks retries.default the same way DynamoDbClient does: DynamoDbClient::_defaultRetries when the flag is on, 11 when it is off.

Testing:

tests/Retry/V3/RoutingTest.php: 6 new cases fail without the source changes; 24/24 pass with them. Full unit test suite also passes.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.