Fix Iceberg incremental read option conflict detection

[fallintoplace]

Summary

• normalize incremental-read and time-travel option keys before comparing them
• detect conflicts for version_ref, branch, and tag reader options
• add parameterized coverage for casing and hyphen/underscore variants

DataFrameReader.option() stores keys in uppercase, while the incremental-read detector compared those keys against lowercase values. As a result, the detector returned early and allowed time-travel options to be silently ignored when incremental reads were configured.

Testing

• python -m pytest tests/unit/test_utils.py -q
• pre-commit run --files src/snowflake/snowpark/dataframe_reader.py tests/unit/test_utils.py

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[fallintoplace]

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