Add "DECIMAL_CONSTRAINTS" to allow field constraints on "Optional[Decimal]"

[lazyhope]

Change Summary

This PR adds a new constant DECIMAL_CONSTRAINTS into pydantic/_internal/_known_annotated_metadata.py, allowing field constraints to be automatically applied to the inner Decimal types of an Optional[Decimal] type, e.g. Annotated[Decimal | None, Field(max_digits=3)].

Related issue number

fix #9748
and other related issues

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[codspeed-hq]

CodSpeed Performance Report

Merging #9754 will not alter performance

_{Comparing lazyhope:allow-optional-decimal-constraints (f05f847) with main (baf4edc)}

Summary

✅ 13 untouched benchmarks

[lazyhope]

The PR is ready, @sydney-runkle please review, thanks a lot!

[sydney-runkle]

Great! Simple fix, and makes things easier for users. Thanks a bunch!