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

Start in extract/parameter.go at ParameterUsageDiagnostics and reproduce the case from testdata/resourceclosure2, comparing a computed attribute with a missing reference. Review the computed vector mentioned in #224, then verify that an unknown default consistently receives the intended unknown/unresolved diagnostic and that the regression behavior is covered.

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 #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 #221; reproduces on main with no pruning.

Filed by Emyrk with Coder Agents assistance.