Allow undefined fields on JsonObject

[skiplabsdaniel]

Today, defining an object with optional values involves the systematic use of the null value or defining the union of all versions of the type.

[mbouaziz]

Review

Note: this review was written entirely by me (Claude Code), not by Mehdi.

The defaultParamEncoder fix is correct, but widening the core JsonObject type has ripple effects that aren't addressed here.

Concerns with adding undefined to JsonObject

• deepFreeze() iterates Object.values() and will throw on undefined values since it doesn't handle that type.
• JSON.stringify() silently drops undefined — ObjectHandle.toString() uses JSON.stringify(this.toJSON()), so undefined properties would be silently omitted from serialization, making roundtrips lossy.
• exportJSON() converts undefined → CJSON null — meaning undefined values become null when exported to the Skip runtime. This is a silent lossy conversion.
• clone() — Object.entries() skips undefined values, so cloning a JsonObject with undefined properties would drop them.

Only one consumer is updated

The PR updates defaultParamEncoder but other places that iterate over JsonObject values would need similar guards or would silently misbehave.

Suggestion

If the goal is to support optional fields, a narrower approach (e.g. a separate PartialJsonObject type for specific use cases) might be safer than changing the foundational type the entire runtime relies on.