fix(agent): VariableAggregator crashes on string variable selectors accepted elsewhere #19412

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Describe your problem

VariableAggregator accepts variable selectors in two forms. VariableAggregatorParam.param_refs (used by the canvas for dependency ordering) handles both:

[selector.get("value") if isinstance(selector, dict) else selector for ...]

But _invoke indexes selector["value"] unconditionally:

for selector in group.get("variables", []):
    val = self._canvas.get_variable_value(selector["value"])

A canvas built through the SDK/API with plain string selectors ("variables": ["a@x"]) parses and orders correctly, then crashes at runtime:

TypeError: string indices must be integers

Reproduction

param.groups = [{"group_name": "G", "variables": ["a@x"]}]
component._invoke()
# TypeError: string indices must be integers

Expected behavior

Both selector forms resolve at runtime, consistent with param_refs.

Version

current main (commit d5ebaf6)