Python: Fix stale agent.json reference in A2A sample docstring

[amit12cool]

Motivation & Context

The A2A hosting sample used /.well-known/agent.json in three places, but the
correct A2A specification path is /.well-known/agent-card.json. These stale
references made the sample broken and misleading out of the box.

Description & Review Guide

• What are the major changes?
  • python/samples/04-hosting/a2a/a2a_server.http: corrected the agent card
    GET endpoint for all three agents (hostInvoice, hostPolicy,
    hostLogistics) from /.well-known/agent.json to
    /.well-known/agent-card.json.
  • python/samples/04-hosting/a2a/a2a_server.py: corrected the module
    docstring and the startup print statement that both referenced the old
    /.well-known/agent.json path.
• What is the impact of these changes?
  Running the A2A sample now works end-to-end: agent card discovery GET
  requests succeed, and the URL printed to the console on startup matches the
  actual endpoint.
• What do you want reviewers to focus on?
  Confirm no other files in the sample still reference the old agent.json path.

Related Issue

Fixes #7286

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 is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• 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]

Pull request overview

Updates the Python A2A hosting sample to consistently reference the current AgentCard endpoint (/.well-known/agent-card.json) instead of the stale /.well-known/agent.json path.

Changes:

• Updated the A2A server sample’s top-of-file documentation string to reference /.well-known/agent-card.json.
• Updated the sample’s startup console output to print the agent-card.json endpoint.
• Updated the accompanying .http sample requests to query /.well-known/agent-card.json.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
python/samples/04-hosting/a2a/a2a_server.py	Aligns in-file docs and the startup “Agent card” URL with the agent-card.json endpoint.
python/samples/04-hosting/a2a/a2a_server.http	Aligns sample HTTP requests with the agent-card.json endpoint.