fix(sdk): add max_spans limit to InMemorySpanExporter (#5296)

* fix(sdk): add max_spans limit to InMemorySpanExporter

* test(sdk): improve test coverage and modernize type hints in InMemorySpanExporter

- Update InMemorySpanExporter type hint from typing.Optional[int] to int | None.
- Refactor test_max_spans_limit to test distinct spans and assert order.

* Apply suggestion from @xrmx

* Create 5296.added

* Rename 5296.added to .changelog/5296.added

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>