Fix precision loss in large integral string conversions

[fallintoplace]

Summary

Fixes precision loss when converting large integral strings in two runtime paths:

• StringLiteral.to(IntegerType/LongType)
• partition_to_py(...) for integral and time-based partition values backed by integers

Root cause

Both paths were converting through float before converting to int, which loses precision for values outside the IEEE-754 exact integer range.

That caused valid 64-bit integers like LongType.max and 9007199254740993 to be corrupted.

What changed

• Replaced int(float(...)) with exact integer parsing in partition_to_py
• For StringLiteral.to(IntegerType/LongType), exact integral strings now use exact integer parsing while fractional numeric strings retain the existing truncation behavior
• Added regression tests for LongType.max and 9007199254740993

Validation

• uv run pytest tests/expressions/test_literals.py tests/test_conversions.py

Closes #3404.