Python: Parse fenced JSON from the group chat orchestrator agent

[Shivam Sharma (ShivamSharma43)]

Motivation & Context

AgentBasedGroupChatOrchestrator asks the orchestrator agent for an AgentOrchestrationOutput via response_format. Providers that don't honor response_format (for example Claude on Bedrock through an OpenAI-compatible Chat Completions endpoint) return the JSON as plain text wrapped in a Markdown code fence. _parse_agent_output can't decode fenced JSON, so the orchestration decision, including terminate: true, is never applied and the group chat runs until max_rounds.

Description & Review Guide

• What are the major changes?
  • _parse_agent_output now also tries the body of the last Markdown code fence (```json, a bare ```, or any language tag) as a parse candidate.
  • Fenced candidates are appended after the raw text, so responses that already parsed, including the concatenated-JSON stop-gap, behave exactly as before.
  • New tests: an end-to-end group chat whose manager returns fenced JSON and terminates on it, a parametrized parser test covering fence variants (language tag, bare fence, uppercase tag with CRLF, single-line fence, prose before the fence), and a check that fenced non-JSON still raises the same ValueError.
• What is the impact of these changes? Group chats with an orchestrator agent on providers that ignore response_format now honor the manager's decisions instead of degrading to max_rounds. No public API changes.
• What do you want reviewers to focus on? Whether using the last fenced block is the right choice when a response contains several (this matches the existing "last JSON object" behavior of _parse_last_json_object).

This covers suggested fix (1) from the issue. Fixes (2), exposing retry_attempts on GroupChatBuilder, and (3), documentation, are left for separate changes.

Related Issue

Fixes #8814

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.

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