fix(workflows): reject a fan-in wait_for that names an unknown step at validation (#3225)

* fix(workflows): reject a fan-in wait_for that names an unknown step at validation

* fix(workflows): reject fan-in wait_for self-reference and non-string entries

Address review feedback on the fan-in wait_for validator:

- A fan-in's own id is added to seen_ids before the wait_for check, so
  `wait_for: [<self>]` passed validation while producing a silent empty
  join at runtime. Reject self-references explicitly.
- Non-string entries (e.g. YAML `wait_for: [123]`) were skipped by the
  isinstance(str) guard and validated even though they can never match a
  real step id. Flag them as wiring errors.

Add coverage for both cases.

src/specify_cli/workflows/engine.py

@@ -296,6 +296,40 @@ def _validate_steps(
                     f"boolean, got {type(coe).__name__}."
                 )
 
+        # Fan-in: every wait_for id must reference a step declared at or before
+        # this point. An id not yet seen is either a typo (unknown step) or a
+        # forward reference (the target runs after this fan-in, so its results
+        # cannot exist yet) — both are wiring errors that previously surfaced as
+        # a silent empty result + COMPLETED. A step that is declared but only
+        # conditionally executed (e.g. inside an if/switch branch) is still
+        # "seen" here, so a legitimately-empty result at runtime stays valid.
+        if step_type == "fan-in":
+            wait_for = step_config.get("wait_for")
+            if isinstance(wait_for, list):
+                for wid in wait_for:
+                    if not isinstance(wid, str):
+                        # A non-string entry (e.g. YAML `wait_for: [123]`) can
+                        # never match a real step id, so the join is silently
+                        # empty at runtime — surface it as a wiring error.
+                        errors.append(
+                            f"Fan-in step {step_id!r}: 'wait_for' entries must "
+                            f"be step-id strings, got {type(wid).__name__} "
+                            f"({wid!r})."
+                        )
+                    elif wid == step_id:
+                        # The fan-in's own id is already in seen_ids by now, so
+                        # a self-reference would pass the membership check below
+                        # while still producing an empty join at runtime.
+                        errors.append(
+                            f"Fan-in step {step_id!r}: 'wait_for' references "
+                            f"itself; a fan-in cannot wait for its own results."
+                        )
+                    elif wid not in seen_ids:
+                        errors.append(
+                            f"Fan-in step {step_id!r}: 'wait_for' references "
+                            f"unknown or not-yet-declared step id {wid!r}."
+                        )
+
         # Recursively validate nested steps
         for nested_key in ("then", "else", "steps"):
             nested = step_config.get(nested_key)