feat(matrix): support outbound attachments via message tool

- extend message tool with optional media paths for channel delivery

- switch Matrix uploads to stream providers and handle encrypted-room payloads

- add/expand tests for message tool media forwarding and Matrix upload edge cases

tests/test_message_tool.py

@@ -0,0 +1,37 @@
+import pytest
+
+from nanobot.agent.tools.message import MessageTool
+from nanobot.bus.events import OutboundMessage
+
+
+@pytest.mark.asyncio
+async def test_message_tool_sends_media_paths_with_default_context() -> None:
+    sent: list[OutboundMessage] = []
+
+    async def _send(msg: OutboundMessage) -> None:
+        sent.append(msg)
+
+    tool = MessageTool(
+        send_callback=_send,
+        default_channel="test-channel",
+        default_chat_id="!room:example.org",
+    )
+
+    result = await tool.execute(
+        content="Here is the file.",
+        media=[" /tmp/test.txt ", "", "   ", "/tmp/report.pdf"],
+    )
+
+    assert result == "Message sent to test-channel:!room:example.org"
+    assert len(sent) == 1
+    assert sent[0].channel == "test-channel"
+    assert sent[0].chat_id == "!room:example.org"
+    assert sent[0].content == "Here is the file."
+    assert sent[0].media == ["/tmp/test.txt", "/tmp/report.pdf"]
+
+
+@pytest.mark.asyncio
+async def test_message_tool_returns_error_when_no_target_context() -> None:
+    tool = MessageTool()
+    result = await tool.execute(content="test")
+    assert result == "Error: No target channel/chat specified"