[BUG] cache read/write usage not exported as string when it is zero

[alesanfra]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.10.0

Python Version

3.12.11

Operating System

macOS 15.7.1

Installation Method

pip

Steps to Reproduce

agent = Agent(
    ...
    model=BedrockModel(
        ...
        cache_prompt="default",
        cache_tools="default",
    )
)

Expected Behavior

{
    "gen_ai.usage.prompt_tokens": "177"
    "gen_ai.usage.input_tokens": "177"
    "gen_ai.usage.completion_tokens": "233"
    "gen_ai.usage.output_tokens": "233"
    "gen_ai.usage.total_tokens": "9944"
    "gen_ai.usage.cache_read_input_tokens": "9534"
    "gen_ai.usage.cache_write_input_tokens": "0"
}

Actual Behavior

{
    "gen_ai.usage.prompt_tokens": "177"
    "gen_ai.usage.input_tokens": "177"
    "gen_ai.usage.completion_tokens": "233"
    "gen_ai.usage.output_tokens": "233"
    "gen_ai.usage.total_tokens": "9944"
    "gen_ai.usage.cache_read_input_tokens": "9534"
    "gen_ai.usage.cache_write_input_tokens": {
        intValue: 0
    }
}

Additional Context

I discovered the issue analyzing the traces with Langfuse:

Possible Solution

No response

Related Issues

No response