tool.execute.before: output.args mutations not propagated to tool execution #26910

Summary

Mutating output.args in a tool.execute.before hook does not affect the tool's actual behavior. The mutation is visible to the plugin code (can read back what was set), but the tool ignores the modified values.

Steps to reproduce

1. Register a tool.execute.before hook
2. In the hook, mutate output.args — e.g. set output.args.model = { providerID: "X", modelID: "Y" }
3. Observe: the hook can read back the mutation, and side-effects (toasts, logs) work fine
4. Check what the tool actually does — the tool ignores the mutated args and uses defaults

Expected

Mutations to output.args in tool.execute.before should propagate to the tool execution. Without this, the hook is effectively read-only — it can observe and notify, but cannot influence tool behavior.

This is a general issue, not specific to any particular field. Any plugin that needs to override tool parameters via this hook will be broken.