Register agent tasks behind use_agent_identity

[adrian-openai]

Stack

• PR1: #15587 Add use_agent_identity feature flag
• PR2: #15588 Register agent identities behind use_agent_identity
• PR3: #15589 Register agent tasks behind use_agent_identity <- this PR
• PR4: #15590 Use AgentAssertion downstream behind use_agent_identity

Summary

• add per-thread agent task registration behind use_agent_identity
• decrypt the returned task id locally and cache the plaintext task for the thread lifetime
• keep this PR scoped to task minting and session-state storage so downstream assertion usage stays isolated

Testing

• stack validation was run on the top branch after the full flow landed