support for bedrock prompt caching

[eluo28]

Please read this first

• Have you read the custom model provider docs, including the 'Common issues' section? Model provider docs
• Have you searched for related issues? Others may have faced similar issues.

Describe the question

I don't believe prompt caching through bedrock as the model provider is supported, I tested the same prompt using bedrock converse API directly which returned prompt cache tokens used as well as using the agents sdk
but the agents sdk never returned >0 cached tokens when called again right after.

Debug information

• Agents SDK version: (e.g. v0.0.3)
• Python version (e.g. Python 3.10)

Repro steps

Ideally provide a minimal python script that can be run to reproduce the issue.

agent = Agent(
    name="big prompt agent",
    instructions= "some prompt that needs prompt caching > token requirement",
    model=LitellmModel(
        model=f"bedrock/{BedrockModelIdentifier.CLAUDE35_HAIKU}",
    ),
)


result = Runner.run_sync(agent, prompt)

# Method 1: Get total usage from context wrapper
total_usage = result.context_wrapper.usage
print("First request usage:")
print(
    total_usage.input_tokens_details,
    total_usage.output_tokens_details,
    total_usage.input_tokens,
    total_usage.output_tokens,
)


result2 = Runner.run_sync(agent, prompt)

print("\nSecond request usage (should show cached tokens):")
total_usage2 = result2.context_wrapper.usage
print(
    total_usage2.input_tokens_details,
    total_usage2.output_tokens_details,
    total_usage2.input_tokens,
    total_usage2.output_tokens,
)

Expected behavior

A clear and concise description of what you expected to happen.

total_usage2 input_tokens_details to return cached_tokens >0