Fold positive Decimal exponents into the mantissa.

[agrawal-priyank]

Summary

• In PyDecimal, treat a positive exponent as trailing zeros on the mantissa and emit DECIMAL(width, 0), instead of using the exponent as a DECIMAL scale. That scale cancelled the 10^n multiply and stored only the mantissa (Decimal('1E+2') → 1; 1E+20 could also overflow int64 and flip the sign).
• Leave fractional Decimals and Arrow/SQL casts unchanged.
• Add round-trip and insert regression tests from the issue, and update the sqlite/pandas cases that encoded the old 1E5 → 1 behavior.

Fixes #566

Test plan

• uv run pytest tests/fast/types/test_decimal.py -v
• uv run pytest tests/fast/sqlite/test_types.py -v
• uv run pytest tests/fast/pandas/test_df_object_resolution.py -v
• uv run pytest tests/fast/test_value.py -v
• CI on this PR