Python: Align A2A hosting README with SDK v1

[LI (ktz03)]

Motivation & Context

The A2A package README still imports the removed a2a.server.apps module in its hosting example. With the supported A2A SDK v1 dependency, that block stops at ModuleNotFoundError before the application can be constructed. The current handler also requires an agent_card.

Description & Review Guide

• What are the major changes? Use the SDK v1 route factories with Starlette, pass the required agent card to the handler, and document the SDK HTTP server extra.
• What is the impact of these changes? Documentation only. On Windows with current Core/A2A source and cached A2A SDK 1.1.4/Starlette 1.6.0, the exact revised block constructs a real executor, handler, app and agent-card/JSON-RPC routes. The unchanged client constructor and async exit also work. The old block stops at its first import; later lines were not executed. No server, ASGI request, remote agent, authentication, model, fresh install, minimum-version matrix, package build or full suite was run.
• What do you want reviewers to focus on? The hosting snippet uses the same route wiring as the existing package guide and hosting sample, while the client example and other README sections remain unchanged.

Related Issue

N/A - trivial change. This is an obvious documentation correction for the already merged SDK v1 migration in #5752. It preserves giles17's runtime and sample work in #5752 and #6165; no implementation is changed.

AI Assistance

• No material AI assistance was used.
• This is an AI-assisted contribution. I reviewed, understood, and verified all submitted content and accept responsibility for it.

AI assistance details: Documentation, example verification, and this PR description.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

🟢 Approval recommended

The focused documentation changes match established hosting patterns, with no unresolved findings.

No open issues 🎉

What changed in this PR

Updates the Python A2A hosting README to match the SDK v1 migration without changing runtime code.

Changes:

• Replaces the removed application helper with Starlette and SDK route factories.
• Passes the required agent card to the request handler.
• Documents installation of the SDK HTTP server extra.

File	Description
python/​packages/​a2a/​README.md	Aligns hosting instructions and example code with SDK v1.

_{Review effort: Balanced}