fix(workflows): reject non-list input 'enum' instead of crashing

[Noor-ul-ain001]

Problem

A workflow input enum is never type-checked. A scalar value crashes the validator and the runtime input resolver with a raw TypeError instead of producing a clean error:

inputs:
  scope:
    type: string
    enum: 5        # or `true`, `3.14`, ...

TypeError: argument of type 'int' is not a container or iterable

The crash comes from the value not in enum_values membership test in WorkflowEngine._coerce_input. Two paths are affected:

1. Authoring time — validate_workflow() documents a "return errors, never raise" contract, but the TypeError escapes its except ValueError and propagates.
2. Run time — execute() does not auto-validate the definition (see load_workflow's docstring), so _resolve_inputs hits the same crash when a provided value is coerced.

A bare-string enum (enum: "abc") is silently worse: value in "abc" becomes a substring/character test rather than enum membership, so it passes validation while doing the wrong thing at runtime.

Fix

Require enum to be a list in both places:

• _coerce_input (the single runtime choke point) raises a clean ValueError on a non-list enum.
• The authoring-time inputs loop in validate_workflow reports it as a validation error — needed independently because _coerce_input is only reached from validation when a default is present, and the integration: auto case strips enum before coercing.

The redundant "invalid default" report is suppressed when enum is already flagged, so the operator sees one clear error rather than the same problem re-framed twice.

This is the same unvalidated-config crash class as the existing switch / fan-in / fan-out non-list guards.

Tests

Added to tests/test_workflows.py:

• scalar enum (no default) → clean validation error, no crash
• string enum → rejected + reported exactly once (no duplicate)
• scalar enum at run time via _resolve_inputs → clean ValueError, not TypeError
• valid list enum → still validates and still checks the default against membership

Full test_workflows.py passes except the pre-existing Windows-without-elevation symlink-guard tests (unrelated to this change).

🤖 Generated with Claude Code