Fix TypeError in os.fork() for garbage-collected processors (#5453)

* Fix NoneType error on os.fork after GC for processors

Assisted-by: Claude Opus 4.6

* Fix regression tests to correctly capture unraisablehook output in child process

* docs: add changelog fragment for fork gc fix

Assisted-by: Google Antigravity

* docs: rename changelog fragment to match PR number

Assisted-by: Google Antigravity

* style: fix trailing whitespace in tests

Assisted-by: Google Antigravity

* test: disable pylint logging-too-few-args in test_export

Assisted-by: Google Antigravity

* Update opentelemetry-sdk/src/opentelemetry/sdk/_shared_internal/__init__.py

Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>

* Update opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/export/__init__.py

Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>

* test: improve robust cleanup and skip logic in fork tests

Assisted-by: Google Antigravity

* test: fix assert usage in test_batch_processor

Assisted-by: Google Antigravity

* style: fix trailing whitespace in tests again

Assisted-by: Google Antigravity

* Fix skip condition in batch processor fork test to also require register_at_fork

Matches the equivalent metrics reader test and addresses Copilot review feedback on PR #5453.

* Update opentelemetry-sdk/tests/metrics/test_periodic_exporting_metric_reader.py

Co-authored-by: Emídio <9735060+emdneto@users.noreply.github.com>

* Fix NameError in test and restore pylint disable comment

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Co-authored-by: Lukas Hering <40302054+herin049@users.noreply.github.com>
Co-authored-by: Emídio <9735060+emdneto@users.noreply.github.com>