docs(a2a): use symbol references in ADK guide

[FanouZeng-TT]

Description

a2a/docs/02-adk-agent.md points readers to several fixed agent.py line
numbers for the ADK agent hooks and configuration:

# agent.py:437
# agent.py:379
# agent.py:408
The agent uses a single instruction (`agent.py:441-454`)
To change the model, edit `agent.py:437`

Those numbers have drifted from the current source: after_tool_modifier is at
line 366, modify_output_after_agent is at line 395, and root_agent starts at
line 424. The referenced symbols are still stable, so the guide can navigate by
symbol name instead of by brittle line numbers.

Fix: replace the stale line-number references with root_agent,
after_tool_modifier, and modify_output_after_agent symbol references.

Category (Required)

• Core Protocol: Changes to the core protocol specification, schemas, or protocol documentation. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance, contribution processes, or project policies. (Requires Governance Council approval)
• Capability: New or modified capability specifications. (Requires Maintainer approval)
• Documentation: Documentation-only changes, website updates, examples, or guides. (Requires Maintainer approval)
• Infrastructure: CI/CD, build systems, tooling, or repository configuration. (Requires DevOps Maintainer approval)
• Maintenance: Dependency updates, refactoring, cleanup, or routine maintenance. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool. (Requires Maintainer approval)
• Community Health (.github): Issue templates, PR templates, code of conduct, or community files. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide and Code of Conduct.
• I have performed a self-review of my changes.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

Validation:

NO_STALE_AGENT_LINE_REFS
source symbols: after_tool_modifier, modify_output_after_agent, root_agent, instruction
npx prettier --check ../docs/02-adk-agent.md: passed
git diff --check: passed
uvx pre-commit run --all-files: passed