Python : Support FakeChatClient

[andresliszt]

Motivation and Context

Description

Thanks for this amazing project. This PR supports #5568. WIP!

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

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[Copilot]

Pull request overview

Adds a deterministic FakeChatClient to the Python core SDK to support testing/demo scenarios without a real model dependency (as requested in #5568).

Changes:

• Introduces agent_framework.fake.FakeChatClient (and FakeChatOptions) for queued/cycling fake responses and basic streaming behavior.
• Adds unit tests covering init/serialization, middleware integration, cycling/exhaustion behavior, and streaming finalization.
• Updates python/PACKAGE_STATUS.md to include a new “fake” package entry.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 5 comments.

File	Description
python/packages/core/agent_framework/fake/_chat_client.py	Implements FakeChatClient and options, including response selection, cycling, and streaming updates.
python/packages/core/agent_framework/fake/init.py	Exposes the fake client/options from the agent_framework.fake namespace.
python/packages/core/tests/core/test_fake_chat_client.py	Adds test coverage for FakeChatClient behavior (middleware, streaming, exhaustion, cycle).
python/PACKAGE_STATUS.md	Adds an entry for an “agent-framework-fake” package.