Correct error message for invalid output types with strict JSON schema (#1101)

### Problem
1. **Incorrect parameter reference**: Suggests passing
`output_schema_strict=False` to `Agent()`, but this parameter doesn't
exist
2. **Vague guidance**: The phrase "make the output type strict" doesn't
explain HOW to make types strict

### Solution
Updated the error message to provide accurate, actionable guidance:

**Before:**
```
"Either make the output type strict, or pass output_schema_strict=False to your Agent()"
```

**After:**
```
"Either use a dataclass, Pydantic model, or TypedDict, or wrap your type with AgentOutputSchema(your_type, strict_json_schema=False)"
```

---------

Co-authored-by: Kazuhiro Sera <[email protected]>

Author: DanielHashmi

src/agents/agent_output.py

@@ -115,8 +115,8 @@ def __init__(self, output_type: type[Any], strict_json_schema: bool = True):
             except UserError as e:
                 raise UserError(
                     "Strict JSON schema is enabled, but the output type is not valid. "
-                    "Either make the output type strict, or pass output_schema_strict=False to "
-                    "your Agent()"
+                    "Either make the output type strict, "
+                    "or wrap your type with AgentOutputSchema(your_type, strict_json_schema=False)"
                 ) from e
 
     def is_plain_text(self) -> bool: