Python: freeze first service_session_id for stable gen_ai.conversation.id across turns (fixes #5992)

[hanhan761]

Summary

Freeze the first non-None service_session_id on AgentSession so that gen_ai.conversation.id remains stable across multi-turn conversations when using the Responses API.

Root Cause

gen_ai.conversation.id was set from session.service_session_id. For the Responses API, service_session_id rotates on every turn (each response returns a new ID used as previous_response_id for the next call). This caused each invoke_agent span to have a different gen_ai.conversation.id, breaking conversation grouping in Application Insights.

Fix

Added a telemetry_conversation_id property to AgentSession that freezes the first non-None service_session_id value. The observability layer now uses this frozen value instead of the current service_session_id, ensuring:

• First turn's service_session_id is preserved for telemetry correlation
• Server-side correlation is maintained (the frozen value is a real server-generated ID)
• Subsequent turn-to-turn ID rotation (Responses API) no longer fragments conversation spans

Issue

Fixes #5992

Verification

• AgentSession.telemetry_conversation_id returns the first non-None service_session_id
• Subsequent service_session_id updates do not change telemetry_conversation_id
• serialization roundtrip preserves service_session_id
• All existing TestAgentSession tests pass (8/8)