[BUG] retry_strategy=None should turn off retries

[mkmeral]

Problem Statement

Currently, in order to disable retry strategy, you need to set max attempts to 1.

Also setting retry strategy to None is equal to the default behavior where we retry models.

    from strands import Agent, ModelRetryStrategy

    agent = Agent(
        retry_strategy=ModelRetryStrategy(max_attempts=1)
    )

Proposed Solution

This is not intuitive, and not a good DevX. As a developer, I'd expect retry_strategy=None to be no retries.

Instead the default should be

def __call__(retry_strategy=ModelRetryStrategy(max_attempts=MAX_ATTEMPTS, max_delay=MAX_DELAY, initial_delay=INITIAL_DELAY)):
    ...

And retry_strategy=None should disable it.

Use Case

DevX

Alternatives Solutions

No response

Additional Context

As we have already released this, changing is backwards incompatible. As a result, we will fix/improve this in v2

[zastrowm]

We should also add API-BarRaising guidance on default values - ideally we'd catch stuff like this before feature merges

[zastrowm]

/strands let's fix this one as a bug; we should use the same strategy as the callback_handler where we use a default sentinel value and if None is passed we create a ModelRetryStrategy that turns off retries