docs(graphs): clarify dynamic node parameter binding

[LeonMAG]

Description

Clarifies the dynamic workflow data-handling example so a function can use a domain-specific parameter name without reading it from workflow state.

With city: str, ADK's default state binding looks for ctx.state["city"] and the workflow fails before the reporting agent runs. The example now uses explicit input binding and passes a dictionary with the matching city key.

Changes

1. Explain the difference between workflow-state binding and named input binding.

2. Configure city_time_function to bind its named input explicitly.

3. Pass matching dictionaries in both uses of the helper and preserve the agent → function → agent flow.

Verification

• mkdocs build --strict --clean
• Verified the binding behavior against the official Google ADK v2.6.3 source: the current scalar form fails with a missing city state value, while explicit input binding maps {"city": "Paris"} to city.

Fixes #2100

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