Fix DynamoDB Rounded error for numbers with trailing zeros

[veeceey]

Summary

Fixes #4693

• Numbers with non-significant trailing zeros (e.g. 1234567895171680000000000000000000000000) cause decimal.Rounded exceptions during serialization and deserialization because Python's Decimal counts trailing zeros as significant digits, exceeding DYNAMODB_CONTEXT's 38-digit precision limit
• The fix normalizes numbers before applying DYNAMODB_CONTEXT to strip non-significant trailing zeros, using a separate _NORMALIZE_CONTEXT with wider precision (100 digits) so normalization itself never silently loses significant digits
• This affects both TypeSerializer._serialize_n and TypeDeserializer._deserialize_n, as well as number sets

Reproduction

from boto3.dynamodb.types import TypeDeserializer

deserializer = TypeDeserializer()
# This raises decimal.Rounded before the fix:
deserializer.deserialize({'N': '1234567895171680000000000000000000000000'})

The number 1234567895171680000000000000000000000000 has only 16 significant digits but its string representation has 40 characters. DynamoDB supports up to 38 digits of precision, so this number is perfectly valid and is accepted by other AWS SDKs (Go, Java, etc.).

Approach

The root cause is that DYNAMODB_CONTEXT.create_decimal(value) treats all digits in the string representation as significant. A value like "1234567895171680000000000000000000000000" is seen as having 40 digits of precision, which exceeds the 38-digit limit and triggers the Rounded trap.

The fix introduces _NORMALIZE_CONTEXT (a decimal.Context with prec=100 and no traps) and calls _NORMALIZE_CONTEXT.normalize(Decimal(value)) before passing to DYNAMODB_CONTEXT.create_decimal(). This converts trailing zeros into an exponent (e.g. 1.23456789517168E+39) so only significant digits count toward the precision check. The wider precision of 100 ensures normalization never rounds values that fit within DynamoDB's 38-digit limit.

Test plan

• All 39 existing tests/unit/dynamodb/test_types.py tests pass unchanged
• All 183 DynamoDB unit tests pass (tests/unit/dynamodb/)
• Added 5 new test cases covering:
  • Serialization of large integers with trailing zeros
  • Serialization of Decimal values with trailing zeros
  • Deserialization of positive numbers with trailing zeros
  • Deserialization of negative numbers with trailing zeros
  • Deserialization of number sets containing values with trailing zeros

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.