fix(langgraph): Unexpected behavior for stream_mode sequences that are not lists (#6354)

## Issue
The `stream_mode` argument type includes `Sequence`, but it doesn't
correctly support non-list sequences. On the other hand, the
`print_mode` argument works as expected.

### Example
```python
from langgraph.pregel.main import Pregel

pregel = Pregel(nodes={}, channels=None, input_channels=[], output_channels=[], auto_validate=False)
stream_modes, *_ = pregel._defaults(
    config={"recursion_limit": 1},
    stream_mode=("values", "messages"),
    print_mode=("values"),
    output_keys=None,
    interrupt_before=None,
    interrupt_after=None,
    durability=None,
)
print(stream_modes) # Expected `{'values', 'messages'}`, got `{('values', 'messages'), 'values'}`
```

libs/langgraph/langgraph/pregel/main.py

@@ -2354,7 +2354,7 @@ class Pregel(
             validate_keys(output_keys, self.channels)
         interrupt_before = interrupt_before or self.interrupt_before_nodes
         interrupt_after = interrupt_after or self.interrupt_after_nodes
-        if not isinstance(stream_mode, list):
+        if isinstance(stream_mode, str):
             stream_modes = {stream_mode}
         else:
             stream_modes = set(stream_mode)