Update AI Agent customization docs for conversation storage SPI breaking changes

[maff]

The conversation storage SPI for the AI Agent connector is being redesigned (camunda/connectors#7021). This introduces breaking changes for custom ConversationStore implementations.

The following page needs to be updated to reflect the new SPI:

• https://docs.camunda.io/docs/components/connectors/out-of-the-box-connectors/agentic-ai-aiagent-customization/

Changes to document:

• New factory-based session pattern (createSession) replacing the callback-based executeInSession
• Decoupled session interface (works with plain message types instead of RuntimeMemory)
• New job completion callback hooks (onJobCompleted, onJobCompletionFailed) on ConversationStore
• Migration guide for existing custom implementations

Related PRs:

• refactor(agentic-ai): redesign conversation storage SPI connectors#6784 (SPI redesign)
• feat: update Camunda Client API to 8.10 and support job completion listeners connectors#6994 (completion callbacks)