fix(ast): avoid panic in ConstantNode.String on non-JSON floats

[netliomax25-code]

ConstantNode.String panics when json.Marshal rejects the value, which happens for NaN and ±Inf. Constant folding produces these for array literals like [0/0], and predicateCombination calls String on the collection argument while optimizing, so any([0/0], # > 0) || any([0/0], # > 0) makes expr.Compile panic instead of returning a program. Compile does not recover, so the panic reaches the host. Fall back to fmt for values json cannot encode.

[netliomax25-code]

any update?

[sanmaxdev]

I checked the non-JSON float path through ConstantNode.String() and the optimizer regression cases. The fallback keeps the existing JSON output for marshalable constants while preventing the NaN/Inf panic path.

Verified locally:

• go test ./ast ./optimizer
• go test ./...
• git diff --check