refactor(sdk): make sure the input is a durable input

[leandrodamascena]

closes #187

Description of changes:

This PR improves the error message when a durable function is invoked with a non-durable payload. Previously, users would see a confusing KeyError: 'DurableExecutionArn' that didn't explain the problem or how to fix it.

Now, when the payload conversion fails, the SDK catches the error and provides a message:

ValueError: The function is not being invoked as a durable function. Please check the function configuration to ensure durability is turned on, or use the testing SDK for local testing.

Why am I using try/except?

The solution wraps the payload conversion in a try/except block that catches KeyError, TypeError, and AttributeError. I think it is more secure than checking for specific keys in the JSON because:

• It validates the entire payload structure during conversion, not just the presence of specific keys
• It catches any malformed or tampered payloads, including wrong data types and malformed nested structures
• It's resilient against attempts to bypass validation by providing partial payloads with the right keys but wrong values
• It handles edge cases that simple key checking would miss

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[ghost]

Thanks! LGTM

[leandrodamascena]

Hey @yaythomas thanks for the review. I left 2 comments.