[Feature]: Redact agent and MCP secrets from Agentcore output

### Description

AgentCore injects agent credentials and MCP secrets into CLI environment variables. Agent or tool output may accidentally expose these values through stdout or stderr.

Add per-invocation secret redaction before output reaches:

- CloudWatch/container logs
- DynamoDB execution output
- WebSocket clients
- Error messages and captured stderr

Requirements:

- Redact resolved agent credentials and MCP secret values.
- Handle secrets split across stream chunks.
- Recursively redact structured output.
- Ensure stdio: inherit cannot bypass redaction.
- Add CloudWatch Logs data-protection policies as defense in depth.
- Add tests for stdout, stderr, structured output, and chunk boundaries.

Redaction protects against accidental disclosure. It is not a security boundary against deliberate encoding or exfiltration by an unrestricted agent.

### Use case

Secure logs

### Area

Backend (Lambda)

### Additional context

_No response_

Start by tracing the AgentCore invocation output paths in the Lambda backend, including stdout, stderr, CloudWatch/container logs, DynamoDB execution output, WebSocket responses, and error messages. Add or locate tests covering direct and chunked secrets, structured output, stdio inheritance, and CloudWatch data-protection policies; done means all listed sinks redact resolved agent credentials and MCP secrets.

Written by the indexing model from the issue text.