stacks: Fix unknown provider values crashing validation phase (#35155)

* test to reproduce

* stacks: Fix unknown provider values crashing validation phase

* remove extra type information

---------

Co-authored-by: CJ Horton <cjhorton@hashicorp.com>

internal/stacks/stackruntime/internal/stackeval/component_config.go

@@ -336,6 +336,12 @@ func (c *ComponentConfig) CheckProviders(ctx context.Context, phase EvalPhase) (
 				})
 				continue
 			}
+		} else if result.Value == cty.DynamicVal {
+			// Then we don't know the concrete type of this reference at this
+			// time, so we'll just have to accept it. This is somewhat expected
+			// during the validation phase, and even during the planning phase
+			// if we have deferred attributes. We'll get an error later (ie.
+			// during the plan phase) if the type doesn't match up then.
 		} else {
 			// We got something that isn't a provider reference at all.
 			diags = diags.Append(&hcl.Diagnostic{

internal/stacks/stackruntime/testdata/mainbundle/test/with-single-input/provider-for-each/provider-for-each.tfstack.hcl

@@ -0,0 +1,32 @@
+required_providers {
+  testing = {
+    source  = "hashicorp/testing"
+    version = "0.1.0"
+  }
+}
+
+variable "provider_set" {
+  type = set(string)
+  default = ["a", "b"]
+}
+
+provider "testing" "configurations" {
+  for_each = var.provider_set
+}
+
+variable "input" {
+  type = string
+}
+
+component "self" {
+  source = "../"
+  for_each = var.provider_set
+
+  providers = {
+    testing = provider.testing.configurations[each.value]
+  }
+
+  inputs = {
+    input = var.input
+  }
+}

internal/stacks/stackruntime/validate_test.go

@@ -61,6 +61,11 @@ var (
 				"input": cty.StringVal("input"),
 			},
 		},
+		filepath.Join("with-single-input", "provider-for-each"): {
+			planInputVars: map[string]cty.Value{
+				"input": cty.StringVal("input"),
+			},
+		},
 	}
 
 	// invalidConfigurations are shared between the validate and plan tests.