[java][bidi] Generated types mishandle special numbers and inbound nulls #18129

Two defects in the generated Java BiDi types, verified on trunk after #17777. Both are Java-only; Python and Ruby already behave correctly.

• NumberValue cannot carry a special number. Inbound {"type":"number","value":"NaN"} throws "value must be one of its declared union types"; outbound toMap() emits the raw SpecialNumber enum instead of "NaN". Regression against the hand-written API for any script.evaluate returning NaN or Infinity. Fix: in appendInlineUnionValidation, coerce a String through the enum arm's fromJson; in serializeExpr, add a union branch that calls toWireValue() on an enum arm. Failing test: NumberValueTest (two cases, both directions).
• Inbound null handling. {"sharedId":"node","handle":null} decodes without error, though handle is not nullable (ADR 17786 decisions 4 and 7). Info.parent set to null and omitted both give parentSet == false, though the SetOverride plumbing exists. Fix: give every generated type its own fromJson, reject a present-but-null non-nullable field there, and pass Optional.of(map.containsKey(wire)) as the SetOverride argument. Keep ConstructorCoercer untouched; it is shared by all of Selenium Java.

Both tests belong in the generator suite proposed in #18128.