fix: prevent shared mutable defaults for span attributes and events

[CagriYonca]

Summary

Fixes #896

This PR resolves an issue where InstanaSpan and ReadableSpan used mutable default arguments (events = [] and attributes = {}), causing all spans created without explicit events/attributes to share the same process-global list. This led to:

• Unbounded memory growth (live Event objects retained indefinitely).
• $O(N)$ CPU overhead on span serialization.
• Cross-trace event/exception leakage in SDK spans.

Changes

• Changed events and attributes default values to None in InstanaSpan.__init__ and ReadableSpan.__init__.
• Updated ReadableSpan.__init__ to instantiate fresh lists/dicts via list(events) and dict(attributes).
• Removed the test fixture workaround (self.span.events.clear()) that previously masked this behavior.
• Added regression unit tests to verify that distinct spans maintain isolated events and attributes.

Testing

• Verified all unit tests in tests/span/ pass.
• Verified isolation and garbage collection against issue [Bug]: Performance: InstanaSpan.__init__ shares one mutable default events list across every span - resulting in unbounded CPU / memory growth #896 reproduction scripts.

[pvital]

It looks good to me.