callback handler - fix reporting of tool when missing delta

[pgrayy]

Description

In the PrintCallbackHandler, we only report a tool if a ToolUseStreamEvent is emitted. This however is only emitted if the model streams tool input deltas. Consequently, if a tool does not have inputs, the model won't produce deltas and thus won't be printed to console. Consider the following example:

from strands import Agent, tool
from strands.models import BedrockModel

@tool
def get_user_info() -> dict:
    """Get user information."""
    return {"name": "Alice", "card": "LIB-456789"}

@tool
def get_book_status(book_id: str) -> dict:
    """Get book status."""
    return {"book_id": book_id, "status": "ACTIVE"}

agent = Agent(
    model=BedrockModel(model_id="qwen.qwen3-next-80b-a3b"),
    tools=[get_user_info, get_book_status],
    system_prompt="You are a helpful assistant."
)

result = agent("Please provide user info and check the status of book BOOK-123")

The result right now is:

Tool #1: get_book_status
User Information:
- Name: Alice
- Card: LIB-456789

Book Status:
- Book ID: BOOK-123
- Status: ACTIVE

With changes in this PR, the output is:

Tool #1: get_user_info


Tool #2: get_book_status
User Information:
- Name: Alice
- Card: LIB-456789

Book Status:
- Book ID: BOOK-123
- Status: ACTIVE

Related Issues

#1551

Documentation PR

N/A

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare: Updated unit tests

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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