fix(agent): improve send_message_to_user tool description to prevent misuse (#6424)

* fix(agent): improve send_message_to_user tool description to prevent misuse

Fixes #6402

The AI was inappropriately using send_message_to_user tool in normal
conversations for text replies, causing duplicate messages (once via tool,
once via normal response).

Root cause: Tool description was not clear enough about when to use it.

Changes:
- Restructure description with clear sections using markdown
- Emphasize two valid use cases:
  1. Sending media files (image, record, video, file) in any conversation
  2. Proactive messaging scenarios (cron jobs, background tasks)
- Add explicit warning: Do NOT use for normal text replies
- Explain consequence: Using for text causes duplicate messages

This approach (better documentation) is safer than restricting tool
registration, which would break media file sending in normal conversations.

Alternative to PR #6413 which was closed due to breaking media delivery.

* fix: correct syntax error and use triple-quoted string for description

Address review feedback:
- P0: Add missing closing parenthesis (was causing SyntaxError on import)
- Use triple-quoted string for better readability (suggested by gemini-code-assist)
- Remove explicit newline characters, let Python handle line breaks

---------

Co-authored-by: ccsang <ccsang@users.noreply.github.com>

astrbot/core/astr_main_agent_resources.py

@@ -83,12 +83,13 @@ class KnowledgeBaseQueryTool(FunctionTool[AstrAgentContext]):
 @dataclass
 class SendMessageToUserTool(FunctionTool[AstrAgentContext]):
     name: str = "send_message_to_user"
-    description: str = (
-        "Send message to the user. "
-        "Supports various message types including `plain`, `image`, `record`, `video`, `file`, and `mention_user`. "
-        "Use this tool to send media files (`image`, `record`, `video`, `file`), "
-        "or when you need to proactively message the user(such as cron job). For normal text replies, you can output directly."
-    )
+    description: str = """Send message to the user. Supports various message types including `plain`, `image`, `record`, `video`, `file`, and `mention_user`.
+
+**IMPORTANT**: This tool is designed for:
+1. Sending media files (`image`, `record`, `video`, `file`) in any conversation
+2. Proactive messaging scenarios (e.g., cron jobs, background task notifications)
+
+**Do NOT use this tool for normal text replies in regular conversations** - just output your text directly instead. Using this tool for text in normal conversations will cause duplicate messages (once via tool, once via normal response)."""
 
     parameters: dict = Field(
         default_factory=lambda: {