fix: correct 'occured' typo in neural type shape mismatch errors

[andrewwhitecdw]

Fixes the spelling of "occurred" in four user-facing shape-mismatch error messages in nemo/core/classes/common.py.

• 2× "Input shape mismatch occured" → "Input shape mismatch occurred"
• 2× "Output shape mismatch occured" → "Output shape mismatch occurred"

Added regression tests that trigger both input and output shape mismatch paths and verify the corrected spelling.

Tested:

uv run python -m pytest tests/core/test_neural_type_shape_mismatch.py -v
# 2 passed

cc @nithinraok for core review

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