fix(tests): apply user edits when creating test from template

The form captured name/description/platform/procedure/tool edits but
never sent them — the created test always used the raw template values.

- TestTemplateInstantiate schema: add optional override fields
  (name, description, platform, procedure_text, tool_used)
- create_test_from_template service: accept *_override kwargs;
  use override value when provided, fall back to template value
- Router: pass all override fields from payload to service
- Frontend API createTestFromTemplate: accept overrides object, spread into body
- TestFromTemplateForm: pass all form state values as overrides

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/test_template.py

@@ -72,7 +72,17 @@ class TestTemplateSummary(BaseModel):
 
 
 class TestTemplateInstantiate(BaseModel):
-    """Payload to create a real test from an existing template."""
+    """Payload to create a real test from an existing template.
+
+    Optional override fields take precedence over the template values when provided.
+    """
 
     template_id: uuid.UUID
     technique_id: str  # accepts both UUID and MITRE ID (e.g. "T1059.001")
+
+    # User-editable overrides (if omitted the template value is used)
+    name: str | None = None
+    description: str | None = None
+    platform: str | None = None
+    procedure_text: str | None = None
+    tool_used: str | None = None