fix(workflows): fan-in validate() rejects non-mapping output (#3349)

FanInStep.validate() only checked wait_for, so a non-mapping 'output'
(a list or scalar) validated clean; execute() then silently coerces it
to {}, so the author's declared aggregation keys vanish with COMPLETED
status and no diagnostic. Reject a non-mapping output at validation,
mirroring the command-step (#3262) non-mapping fix. execute()'s
defensive coercion is left in place for unvalidated callers.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/specify_cli/workflows/steps/fan_in/__init__.py

@@ -58,4 +58,13 @@ class FanInStep(StepBase):
                 f"Fan-in step {config.get('id', '?')!r}: "
                 f"'wait_for' must be a non-empty list of step IDs."
             )
+        output = config.get("output")
+        if output is not None and not isinstance(output, dict):
+            # execute() silently coerces a non-mapping output to {}, so the
+            # author's declared aggregation keys would vanish with no error.
+            # Reject at validation, mirroring the command-step (#3262) fix.
+            errors.append(
+                f"Fan-in step {config.get('id', '?')!r}: 'output' must be a "
+                f"mapping of key -> expression, got {type(output).__name__}."
+            )
         return errors