[Enhancement] Reduce create_agent() calls per invocation in AgentCoreMemorySessionManager

[emmanuelesperanca]

Behavior observed:
AgentCoreMemorySessionManager.create_agent() is called N times per single
agent invocation (once per tool-use cycle in the Strands agent loop),
generating redundant memory write events and increasing latency

Evidence:
Session [X]: create_agent() called 3x for a single-turn interaction
with 1 tool call (KB search), generating events at t+0ms, t+366ms, t+5866ms
within the same traceId

Expected behavior:
create_agent() could be called once per invocation/session initialization? And not once per tool-use cycle?

Proposed solution options:

1. Add a flag create_agent_per_turn: bool = False to AgentCoreMemoryConfig
2. Deduplicate create_agent() calls within the same session_id + agent_id
   using an in-memory set in the session manager

Impact:

• Extra memory write operations per invocation
• Noise in observability traces
• Increased latency for multi-tool invocations