fix(sdk): add max_spans limit to InMemorySpanExporter

[shayannab]

Description

InMemorySpanExporter currently stores all spans in an unbounded list,
which can cause memory issues in long-running test environments or
production debug scenarios.

This PR adds an optional max_spans parameter to __init__. When set,
the internal storage uses collections.deque(maxlen=max_spans) which
automatically drops the oldest spans when the limit is reached. Default
is None (no limit), so existing behaviour is fully preserved.

Fixes #5281

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Ran the existing test suite plus two new tests via tox:
tox -e py312-test-opentelemetry-sdk -- tests/trace/export/test_in_memory_span_exporter.py

• test_max_spans_limit — verifies oldest spans are dropped when limit is reached
• test_max_spans_none_is_unlimited — verifies default behaviour is unchanged

[linux-foundation-easycla]

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• ✅ login: shayannab / name: shayannab (c11f724)