test: add fixture for non-string variable value

Adds test coverage for the case where a YAML number value appears
in the variables record (e.g., `count: 5`). YAML parses unquoted
numbers as integers, which the schema correctly rejects as
invalid_type (expected string, received number). This is a likely
contributor mistake worth covering.

28/28 fixtures now passing.

test/fixtures/workflow-schema/invalid/bad-variables-type.workflow.yaml

@@ -0,0 +1,14 @@
+# Expected: FAIL
+# Error code: invalid_type
+# Error path: variables.count
+# Error expected: string
+# Error received: number
+# Tests: variables with non-string value (YAML number) produces wrong_type error
+name: bad-variables-type-test
+description: "A workflow with non-string variable value"
+author: "Test"
+standalone: true
+web_bundle: false
+variables:
+  valid_key: "string value"
+  count: 5