Document JsonNumber numeric conversion design choices

[cursor]

Problem

Older revisions of this backport included convenience constructors/factories on the core java.util.json types (notably around JsonNumber) that accepted BigDecimal / BigInteger. After the most recent sync from upstream openjdk/jdk-sandbox those APIs are no longer present.

This leaves users wondering:

• what happened to the BigDecimal/BigInteger entry points
• whether their removal was accidental vs. deliberate upstream design
• how to do lossless numeric handling without the constructor explosion typical of legacy OO APIs

Goal

Document the design choice: JSON numbers are represented primarily as lexical decimal text, with minimal numeric factories (double, long, String) and lossless conversion via toString().

Add runnable examples (and tests) showing:

• precision loss with double
• lossless conversion using BigDecimal/BigInteger via toString()
• how to build JsonNumber from BigDecimal without adding new overloads
• how lexically-preserving toString() differs from numeric normalization

Deliverables

• DESIGN_CHOICES.md analysis with examples
• matching example sources under json-java21/src/main/... and json-java21/src/test/...

[simbo1905]

There are some design decisions stuff I was looking into; yet it would be against the stated rules of the repo to debate the java.util.json design in this repo keep it for the core jdk forums.