[Bug]: Validation error example uses non-existent TASK_STATE_RUNNING enum and lowercase enum values

### What happened?

The validation error example in Section 6.5 of the specification has two inconsistencies:

1. **Mismatched error reference**: The request query parameter sends `status=TASK_STATE_RUNNING` but the error message references `'running'` instead of `'TASK_STATE_RUNNING'`

2. **Lowercase enum values**: The error message lists valid values as `pending, working, completed, failed, canceled` (lowercase, incomplete list), but Section 5.5 mandates SCREAMING_SNAKE_CASE per the ProtoJSON specification, and all other spec examples use this format (e.g., `TASK_STATE_WORKING` on line 1494)

Additionally, `TASK_STATE_RUNNING` does not exist in the `TaskState` enum defined in `a2a.proto` (the correct value is `TASK_STATE_WORKING`), but since this is a validation error example showing an intentionally invalid request, the invalid enum is acceptable — the error response should just be consistent with the rest of the spec.

### Relevant log output

```
# Section 5.5 (line 1215) says:
# Enum values MUST be represented as their string names as defined
# in the Protocol Buffer definition (typically SCREAMING_SNAKE_CASE).

# But the validation error example (line 1587) says:
"Must be one of: pending, working, completed, failed, canceled"

# Other examples in the spec correctly use:
status=TASK_STATE_WORKING (line 1494)
"state": "TASK_STATE_WORKING" (line 1511)
```

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