feat: add read_only flag to AgentCoreMemorySessionManager to disable ACM persistence

[mohanasudhan]

Description

Add a read_only option to AgentCoreMemoryConfig that disables persistence to AgentCore Memory (create_event calls) while keeping local session/agent state management and memory injection (LTM retrieval) working.

Motivation

Resolves strands-agents/sdk-python#2020

The AgentCoreMemorySessionManager currently persists agent state and injects memories. Customers who only need memory injection into agent context (without persisting agent state) had no way to opt out of persistence. This adds a read_only=True flag so existing customers can disable persistence with a single config change.

Changes

• config.py: Added read_only: bool = Field(default=False) to AgentCoreMemoryConfig
• session_manager.py: Guarded ACM write calls (create_event, delete_event) behind read_only check while preserving local state management (validation, caching, append_message tracking)
• Tests: 20 new tests covering all read_only behavior

What's guarded (ACM writes skipped when read_only=True)

• create_session — skips create_event, still validates and returns session
• create_agent — skips create_event/buffering, still caches created_at
• update_agent — delegates to create_agent, cache lookup still works
• create_message — skips create_event/buffering, returns {} so append_message tracks local state
• _flush_messages_only / _flush_agent_states_only — no-op
• Legacy migration paths — skips create_event + delete_event, still reads data

What's unaffected

• All read operations (read_session, read_agent, list_messages)
• LTM retrieval (retrieve_customer_context)

Usage

config = AgentCoreMemoryConfig(
    memory_id="my-mem",
    session_id="sess-1",
    actor_id="user-1",
    read_only=True,  # no writes to AgentCore Memory
)
session_manager = AgentCoreMemorySessionManager(agentcore_memory_config=config)

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