fix: correct nested path traversal in set_variable_param_value (#13986)

## Summary

`Graph.set_variable_param_value()` in `agent/canvas.py` has a bug in its
nested path traversal logic. The `for` loop iterates through **all**
keys in the path (including the last one), descending into every level.
After the loop, it then tries to set `cur[keys[-1]] = value`, but `cur`
has already descended one level too deep.

**Example:** For `path = "a.b"`, `value = "hello"`:
- **Before (bug):** `obj["a"]["b"]` becomes `{"b": "hello"}` instead of
`"hello"`
- **After (fix):** `obj["a"]["b"]` becomes `"hello"` as expected

The fix changes `for key in keys:` to `for key in keys[:-1]:`, so the
loop only navigates to the parent dict, and the final key is set
directly. This is consistent with how the read-side counterpart
`get_variable_param_value()` works.

This method is called by `set_variable_value()` when assigning to nested
variable paths (e.g., `component@root.nested.key`), which is used by the
`VariableAssigner` component.

## Test plan
- [ ] Create a canvas with a VariableAssigner that writes to a nested
path (e.g., `component@obj.nested.key`)
- [ ] Verify the value is set correctly at the expected path, not
wrapped in an extra dict layer
- [ ] Verify single-key paths (e.g., `component@key`) still work
correctly

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Fixed a bug in variable parameter assignment where nested structures
were being incorrectly modified, ensuring values are now properly set at
their intended locations without unintended overwrites.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent/canvas.py

@@ -263,7 +263,7 @@ class Graph:
         keys = path.split('.')
         if not path:
             return value
-        for key in keys:
+        for key in keys[:-1]:
             if key not in cur or not isinstance(cur[key], dict):
                 cur[key] = {}
             cur = cur[key]