Unknown parameter default coerced to null skips the 'value is unknown' diagnostic

[Emyrk]

Summary

A parameter whose default is an unknown value can surface either as an error diagnostic or as a silent null, depending on how the unknown arrives. Both cases are "the value could not be resolved", but only one of them tells the user.

Repro

testdata/resourceclosure2 on main (from #224):

resource "docker_image" "base" { name = "large" }

data "coder_parameter" "computed" {
  name    = "computed"
  type    = "string"
  default = docker_image.base.image_id   # computed attribute, unknown at plan time
}

Result on main:

value=cty.NullVal(cty.String) valid=false known=false null=true default=cty.DynamicVal diags=0

DefaultValue is cty.DynamicVal, Value ends up as a typed null, and the parameter carries no diagnostics.

Compare a default that is unknown because a reference has no value (for example default = local.x where local.x reads a block that does not exist). There the value stays an unknown and ParameterUsageDiagnostics emits:

Parameter value is unknown, it likely includes a reference without a value

Where it diverges

extract/parameter.go, ParameterUsageDiagnostics:

if p.Value.Value.IsNull() {
    // Allow null values
} else if !p.Value.Valid() {
    ...
} else if !p.Value.IsKnown() {
    ... "Parameter value is unknown" ...
}

The null branch is checked first, so an unknown default that is coerced to a typed null on the way to Value skips both the "not valid" and "unknown" branches, even though Valid() and IsKnown() are both false on it.

Expected

Either both shapes report the same "unknown / unresolved" diagnostic, or neither does. A user writing default = <computed attribute> gets a required-but-empty parameter in the form with no explanation today.

Notes

• Found while writing test: pin more evaluation paths from parameters to resources #224; the computed vector there pins the current behavior with ap().unknown() and no diagnostic assertion, so it will not block a fix.
• Not related to the resource-closure work in feat: evaluate only the parameter/preset/tag closure when rendering #221; reproduces on main with no pruning.

Filed by Emyrk with Coder Agents assistance.