Preserve Agenttic client context

[lezama]

Summary

• parse Agenttic message data parts carrying clientContext into canonical agents/chat client_context
• pass per-turn client_context into the conversation loop even when no tools are present
• inject client_context into the model prompt ephemerally, without storing it in the transcript

Tests

• php -d auto_prepend_file=tests/phpunit-prepend.php vendor/phpunit/phpunit/phpunit tests/unit/AgentticClientContextTest.php
• composer lint:php
• composer test
• composer test:assembly
• composer validate --strict
• composer audit