fix(gnovm): Add debug panic on Deepfill execution on constant type

[Davphla]

Fix: #4777

DeepFill replace all references (RefValue) with concrete values loaded from storage.

StringValue, BigintValue, and BigdecValue are value types that are never persisted as separate objects. They lack ObjectInfo (ref gnovm/pkg/gnolang/ownership.go:144) and are used for:

• Constant expressions (embedded in ConstExpr nodes during preprocessing)
• Runtime untyped values (e.g., UntypedBigintType operations in gnovm/pkg/gnolang/op_binary.go:693)

Because these types are never persisted separately, they should never appear as RefValue that require DeepFill.
This is confirmed by their VisitAssociated implementations in gnovm/pkg/gnolang/garbage_collector.go:378-386, which return false indicating no associated values to visit during GC.

This PR adds debug-mode assertions that panic when DeepFill is called on these constant-only types, making it easier to catch incorrect handling of constant expressions during development.

[Gno2D2]

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gnovm/pkg/gnolang/values_fill.go	0.00%	3 Missing and 3 partials ⚠️

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[jefft0]

Hi @Davphla . Can you fix the failed CI check for codecov?

[Davphla]

I've added minimal tests. Since added lines cover debug features and CI doesn't run in debug mode, further coverage isn't possible.

[thehowl]

I'd say just make them panic, in correct code this adds no overhead anyway (because DeepFill is often called through an interface, not lending itself to optimization). So might as well always panic.

[thehowl]

Why not just make these panic?

[Davphla]

This may cause involuntary informational panic, so I think it is safer to have a potential optimization then an accidental panic.

As well, it break tests 2b0ae1d
Failing test: https://github.com/gnolang/gno/actions/runs/19742512315/job/56569620776#step:5:10

Should the failing tests be considered in that PR?

[thehowl]

If it's not intended to happen, then it won't be "informational".

But by seeing the test results, it looks like either the behaviour is incorrect in the callers; or it would be incorrect to have a panic here