Python: Allow branching from hosted Foundry conversations

[cecheta]

Motivation & Context

Allow branching from previous response in a conversation, by always storing state for response ID, plus conversation if applicable.

Description & Review Guide

• What are the major changes?
• What is the impact of these changes?
• What do you want reviewers to focus on?

State is now always stored under the response ID. If a conversation is supplied:

• For normal agents, state is also stored under the conversation
• For workflow agents, state is copied from the response ID state to the conversation ID state

Related Issue

Fixes #7524

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.

[github-actions]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/foundry_hosting/agent_framework_foundry_hosting
_responses.py	942	93	90%	199–202, 270, 275, 299, 354–355, 369, 372–373, 405–406, 440, 456, 518, 643, 657, 707, 767, 770, 776, 778, 850, 931, 933, 974–975, 1093, 1323, 1336, 1827–1829, 1831, 1878–1879, 1881–1882, 1884–1885, 1887–1888, 1893, 1902, 1905–1907, 1909, 1919, 1923, 1936, 2027–2028, 2043–2044, 2046, 2051–2055, 2057, 2064–2065, 2067–2068, 2074, 2076–2080, 2087, 2093, 2115, 2121, 2127, 2129, 2131–2134, 2142, 2144, 2205–2207, 2221–2222, 2225–2226
_session_store.py	26	0	100%
TOTAL	44687	4106	90%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
9135	34 💤	0 ❌	0 🔥	2m 3s ⏱️

[Copilot]

Pull request overview

This PR updates the Python Foundry Hosting integration to support conversation branching by ensuring every Responses turn persists state under its immutable response_id, while conversations also maintain a conversation_id latest-state alias for continuation.

Changes:

• Persist regular-agent session snapshots under every response_id, and additionally update the conversation_id snapshot as a “latest turn” alias when a conversation is supplied.
• For workflow agents, write checkpoints per-turn under response_id and update a conversation_id checkpoint alias to the latest checkpoint after each conversation turn.
• Add/extend tests to validate branching behavior for both regular agents and workflow agents (streaming and non-streaming), and update Foundry Hosting docs to reflect the new persistence model.

Show a summary per file

File	Description
python/packages/foundry_hosting/tests/test_responses.py	Adds branching-focused tests for both session snapshots and workflow checkpoints (including streaming).
python/packages/foundry_hosting/README.md	Documents the new model: always store under response_id plus a conversation_id latest-state alias.
python/packages/foundry_hosting/agent_framework_foundry_hosting/_session_store.py	Updates Foundry session store docstring to describe response-first storage plus conversation aliasing.
python/packages/foundry_hosting/agent_framework_foundry_hosting/_responses.py	Implements response-id-first persistence for sessions and checkpoints; introduces workflow checkpoint finalization to update conversation aliases.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite