fix(slack): preserve parent agent's system prompt in message processing

[strands-agent]

Description

The Slack tool was overwriting the agent's system_prompt with only the SLACK_SYSTEM_PROMPT constant, ignoring any custom system prompts provided by the parent agent.

This fix ensures the parent agent's system_prompt is preserved and prepended to the Slack-specific context in:

• _process_message: Both agent creation and system_prompt refresh
• _process_interactive: Both agent creation and system_prompt refresh

The Problem

When a user creates an agent with a custom system prompt:

agent = Agent(
    tools=[slack_send_message],
    system_prompt="It will bark Bow! at first"  # This was being ignored!
)
slack(action="start_socket_mode", agent=agent)

The custom system prompt was lost because the code overwrote it with:

agent.system_prompt = f"{SLACK_SYSTEM_PROMPT}\n\nEvent Context:..."  # Missing parent prompt!

The Fix

Now preserves the parent agent's system prompt:

agent.system_prompt = (
    f"{self.agent.system_prompt}\n{SLACK_SYSTEM_PROMPT}\n\n"
    f"Event Context:..."
)

This follows the pattern already established in agent creation at line 359, now consistently applied across all locations.

Related Issues

Fixes #302

Type of Change

Bug fix

Testing

• Added unit tests for system prompt preservation in test_slack_socket.py
• All existing Slack tests pass
• Verified the fix matches the proposed solution from the issue author

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• My changes generate no new warnings

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