fix(workflows): fail fan-in loudly on a non-string wait_for entry (#3579)

`FanInStep.execute` already guards a non-list `wait_for` (#3482), and the
engine's load-time validation rejects non-string entries. But the engine does
not auto-validate step config, so on an unvalidated run `execute` iterated the
list's *elements* raw:

- An unhashable entry (a list/dict from a YAML indentation slip like
  `wait_for: [[a, b]]`) crashed the whole run at `context.steps.get(entry, ...)`
  with a raw `TypeError: cannot use 'list' as a dict key`.
- A hashable-but-non-string entry (`wait_for: [123]`) silently joined an empty
  `{}` and still reported COMPLETED — the exact "silent empty result +
  COMPLETED" wiring bug the whole-list guard and the engine's fan-in
  validation both exist to prevent.

Extend the execute() guard to reject any non-string entry with the engine's
"entries must be step-id strings" phrasing, mirroring the sibling non-list
guard right above it. Adds regression coverage for unhashable and
hashable-non-string entries.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -42,6 +42,28 @@ class FanInStep(StepBase):
                 output={"results": []},
             )
 
+        # A non-string entry can never match a real step id. An unhashable one
+        # (a list/dict from a YAML indentation slip like ``wait_for: [[a, b]]``)
+        # crashes the whole run at ``context.steps.get(step_id, ...)`` below with
+        # a raw TypeError; a hashable-but-non-string one (``wait_for: [123]``)
+        # silently joins an empty ``{}`` and still reports COMPLETED — the exact
+        # "silent empty result + COMPLETED" wiring bug the whole-list guard above
+        # and the engine's fan-in validation (engine.py) both reject. The engine
+        # does not auto-validate step config, so fail this step loudly on an
+        # unvalidated run too, using the engine's phrasing.
+        bad_entries = [w for w in wait_for if not isinstance(w, str)]
+        if bad_entries:
+            first = bad_entries[0]
+            return StepResult(
+                status=StepStatus.FAILED,
+                error=(
+                    f"Fan-in step {config.get('id', '?')!r}: 'wait_for' entries "
+                    f"must be step-id strings, got {type(first).__name__} "
+                    f"({first!r})."
+                ),
+                output={"results": []},
+            )
+
         # Collect results from referenced steps
         results = []
         for step_id in wait_for: