[Bug] AwsBedrock dict tool arguments raise AttributeError on strip

Description

AwsBedrock request assembly calls arguments.strip() on tool-call arguments. Already-parsed dicts raise AttributeError and abort the Bedrock request. JSON null is handled by the truthiness check, but a dict is not.

Steps to Reproduce

from agno.models.aws.bedrock import AwsBedrock
from agno.models.message import Message

model = AwsBedrock(id="mistral.mistral-small-2402-v1:0", aws_region="us-east-1")
assistant = Message(
    role="assistant",
    content=None,
    tool_calls=[{
        "id": "tooluse_1",
        "type": "function",
        "function": {"name": "get_weather", "arguments": {"city": "London"}},
    }],
)
model._format_messages([Message(role="user", content="hi"), assistant])

Expected Behavior

A dict is used as the tool input. Empty or null arguments become {}. Request assembly succeeds.

Actual Behavior

AttributeError: 'dict' object has no attribute 'strip'

Environment

• OS: Windows 11
• Agno Version: current main
• Additional Environment Details: Python 3.13

Possible Solutions (optional)

Treat dict arguments as already parsed. Keep JSON strings. Treat None or blank strings as {}.