feat: validate presets on template import (#18844)

Typos and other errors often result in invalid presets in a template.
Coder would import these broken templates and present them to users when
they create workspaces. An unsuspecting user who chooses a broken preset
would then experience a failed workspace build with no obvious error
message.

This PR adds additional validation beyond what is possible in the
Terraform provider schema. Coder will now present a more helpful error
message to template authors when they upload a new template version:

<img width="1316" height="286" alt="Screenshot 2025-07-14 at 12 22 49"
src="https://github.com/user-attachments/assets/7f5f778f-d9ae-487a-95e2-f6f1ca604a9c"
/>

The frontend warning is less helpful right now, but I'd like to address
that in a follow-up since I need frontend help:

<img width="1102" height="616" alt="image"
src="https://github.com/user-attachments/assets/e838ffc8-ef4f-428d-9280-74fa0c491666"
/>

closes https://github.com/coder/coder/issues/17333


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Improved validation and error reporting for template presets,
providing clearer feedback when presets cannot be parsed or reference
undefined parameters.

* **Bug Fixes**
* Enhanced error handling during template version creation to better
detect and report issues with presets.

* **Tests**
* Added new tests to verify validation of both valid and invalid
Terraform presets during template version creation.
* Improved test reliability by enabling dynamic control over error
injection in database-related tests.

* **Chores**
* Updated a dependency to the latest version for improved stability and
features.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

coderd/dynamicparameters/error.go

@@ -26,6 +26,14 @@ func tagValidationError(diags hcl.Diagnostics) *DiagnosticError {
 	}
 }
 
+func presetValidationError(diags hcl.Diagnostics) *DiagnosticError {
+	return &DiagnosticError{
+		Message:          "Unable to validate presets",
+		Diagnostics:      diags,
+		KeyedDiagnostics: make(map[string]hcl.Diagnostics),
+	}
+}
+
 type DiagnosticError struct {
 	// Message is the human-readable message that will be returned to the user.
 	Message string

coderd/dynamicparameters/presets.go

@@ -0,0 +1,28 @@
+package dynamicparameters
+
+import (
+	"github.com/hashicorp/hcl/v2"
+
+	"github.com/coder/preview"
+)
+
+// CheckPresets extracts the preset related diagnostics from a template version preset
+func CheckPresets(output *preview.Output, diags hcl.Diagnostics) *DiagnosticError {
+	de := presetValidationError(diags)
+	if output == nil {
+		return de
+	}
+
+	presets := output.Presets
+	for _, preset := range presets {
+		if hcl.Diagnostics(preset.Diagnostics).HasErrors() {
+			de.Extend(preset.Name, hcl.Diagnostics(preset.Diagnostics))
+		}
+	}
+
+	if de.HasError() {
+		return de
+	}
+
+	return nil
+}

coderd/dynamicparameters/tags.go

@@ -11,6 +11,10 @@ import (
 
 func CheckTags(output *preview.Output, diags hcl.Diagnostics) *DiagnosticError {
 	de := tagValidationError(diags)
+	if output == nil {
+		return de
+	}
+
 	failedTags := output.WorkspaceTags.UnusableTags()
 	if len(failedTags) == 0 && !de.HasError() {
 		return nil // No errors, all is good!