Inject converter

[afarntrog]

Issue #, if available:
#254

Description of changes:

• Add OpenAI-format message converter for AgentCore Memory, enabling Strands SessionMessages (Bedrock Converse format) to be stored and retrieved as
  OpenAI-format payloads in STM events. Handles text messages, tool calls (tool_calls), tool results (tool role), and system messages with bidirectional
  conversion.
• Extract converters into a converters subpackage under memory/integrations/strands/, introducing a MemoryConverter protocol that both BedrockConverseConverter
  and OpenAIConverseConverter implement. Shared logic (exceeds_conversational_limit, CONVERSATIONAL_MAX_SIZE) is centralized in protocol.py.
• Make the converter injectable on AgentCoreMemorySessionManager via a new converter parameter (defaults to BedrockConverseConverter), allowing callers to plug
  in OpenAIConverseConverter or any custom implementation that satisfies the MemoryConverter protocol.
• Rename AgentCoreMemoryConverter to BedrockConverseConverter for clarity, and update all references.
• Add comprehensive tests: 504-line test suite for OpenAIConverseConverter covering message-to-payload, events-to-messages, round-trips, edge cases, and error
  handling. Add 143 lines of tests for the injectable converter integration on the session manager (default/custom converter, blob fallback, empty payloads).

Test plan

• Run pytest tests/bedrock_agentcore/memory/integrations/strands/test_openai_converter.py -- all new OpenAI converter tests pass
• Run pytest tests/bedrock_agentcore/memory/integrations/strands/test_bedrock_converter.py -- existing Bedrock converter tests pass with renamed class
• Run pytest tests/bedrock_agentcore/memory/integrations/strands/test_agentcore_memory_session_manager.py -- session manager tests pass, including new
  TestInjectableConverter class

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