test: #3586021 Add regression test for ai_agent plugin cache invalidation

Bharat Kelotrarequested to merge
issue/ai_agents-3564033:3564033-connecting-an-agent-to-an-assistant-doesnt-work-until-cache-clear-and-tool-api into 1.3.x

Adds a kernel test covering the fix in ai_agents_ai_agent_insert()/ _update()/_delete() that clears the plugin.manager.ai_agents and plugin.manager.ai.function_calls caches when an ai_agent config entity is saved or deleted. Reproduces the reported scenario (create an Agent, do not clear caches, explore/attach it) and asserts the agent is immediately usable without a manual cache clear.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

Closes #3564033

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