Fix set_api_key breaking cached property mechanism

[DanielHashmi]

Problem

The BackendSpanExporter.set_api_key() method was directly assigning to self.api_key, which created an instance attribute that permanently shadowed the @cached_property. This broke the environment variable fallback mechanism.

Solution

• Clear the cached property if it exists before updating the private attribute
• This preserves the intended fallback behavior to environment variables

Testing

• Added test to verify environment variable fallback works after calling set_api_key()
• Existing functionality remains unchanged

[seratch]

LGTM; Thank you for sending this!