changed example to plugin

[JuanCaCoder]

Changed the code sample to use the GlobalInstructionPlugin.

[JuanCaCoder]

📝 Executive Summary

• Migrates the Python A2A Consuming Quickstart sample from the deprecated Agent.global_instruction parameter to GlobalInstructionPlugin registered via App(plugins=[...]).
• Aligns the quickstart with the modern ADK application architecture and prevents runtime DeprecationWarning exceptions while preserving global instruction behavior across sub-agents.
• Target doc: docs/a2a/quickstart-consuming.md (rendered under /a2a/quickstart-consuming/).

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🗂️ PR Consolidation & Verification Mapping

PR / Source Reference	Original Section	New Section	Change Description	Codebase Verification
docs/a2a/quickstart-consuming.md	#### How it works (Snippet)	#### How it works (Snippet)	Removed deprecated global_instruction from root_agent = Agent(...); added App(name="a2a_basic", root_agent=root_agent, plugins=[GlobalInstructionPlugin(...)]).	✅ PASS google/adk/plugins/global_instruction_plugin.py:L58-L85 google/adk/apps/app.py:L53-L83
docs/a2a/quickstart-consuming.md	#### How it works (Prose)	#### How it works (Admonition)	Added !!! note "Global instructions via Plugins" note explaining deprecation of Agent.global_instruction and pointing to GlobalInstructionPlugin.	✅ PASS google/adk/agents/llm_agent.py:L758-L766 docs/agents/llm-agents.md:L206-L210
docs/a2a/quickstart-consuming.md	Introductory text	Introductory text	Added link to GlobalInstructionPlugin under /plugins/#prebuilt-plugins.	✅ PASS docs/plugins/index.md:L67-L68

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🔍 Technical Verification Report

1. GlobalInstructionPlugin Implementation & Lifecycle:

   • Source: src/google/adk/plugins/global_instruction_plugin.py:L58-L115
   • Verified: GlobalInstructionPlugin inherits from BasePlugin. Its constructor accepts global_instruction: Union[str, InstructionProvider] = "" and name: str = "global_instruction". In before_model_callback, it resolves static strings or InstructionProvider callables and prepends the instruction to llm_request.config.system_instruction.

2. Agent.global_instruction Deprecation:

   • Source: src/google/adk/agents/llm_agent.py:L288-L299, L758-L766
   • Verified: The field is marked deprecated in docstrings and emits a runtime DeprecationWarning ("global_instruction field is deprecated and will be removed in a future version. Use GlobalInstructionPlugin instead for the same functionality at the App level.").

3. App Container Architecture:

   • Source: src/google/adk/apps/app.py:L53-L83
   • Verified: App requires name: str and root_agent: Union[BaseAgent, Any, None]. It accepts plugins: list[BasePlugin] = Field(default_factory=list), which applies registered plugins application-wide across all local and remote sub-agents.

4. Runner plugins Deprecation:

   • Source: src/google/adk/runners.py:L285-L286, L386-L387
   • Verified: Passing plugins directly to Runner is deprecated in favor of App(plugins=[...]), confirming App as the canonical entry point for plugin registration.