Python: Clarify A2A quickstart working directories - #10177

Motivation & Context

The A2A hosting quickstart starts in the server sample directory, but the client command uses a path relative to the repository root. Following the instructions from the server directory makes the client's cd command fail.

Description & Review Guide

• What are the major changes? Specify that server setup starts at the repository root and that the client runs in a separate terminal opened at the repository root.
• What is the impact of these changes? Readers can follow both sets of commands from the documented working directories.
• What do you want reviewers to focus on? Check that the server and client steps each state their starting directory clearly.

Verified that the original client path fails from the server sample directory and that the documented path resolves to agent_with_a2a.py from the repository root. git diff --check passed. Validation was limited to the documented paths.

Related Issue

N/A - trivial change. This corrects the working-directory instructions in the A2A hosting quickstart.

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: AI identified the working-directory mismatch, drafted the documentation and PR description, and checked the paths locally.

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.