[BREAKING] Python: Ensure session isolation for FHA invocation impl

[TaoChenOSU]

Motivation & Context

Currently, the light-weight Foundry Hosting Agent (FHA) invocation integration stores live AgentSession objects in a process-wide dictionary keyed only by the session_id, which could potentially allow cross-user invocation.

Description & Review Guide

This PR ensures that the sessions are key by the user id injected by the platform when the agent is hosted in FHA., eliminating the possibility that one user can interfere with another user's session even if the session id is exposed.

• What are the major changes?
  • Agent sessions are now keyed by partition keys that are made up of the platform session id and user id, when hosted.
  • When not hosted, session will be isolated like before because user id is not available.
• What is the impact of these changes?
  • Eliminates cross user interference
• What do you want reviewers to focus on?
  • The derivation of the partition key

This is marked as breaking because the return type of the invocation handler has been changed.

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
_invocations.py	45	0	100%
TOTAL	44469	5212	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
8995	33 💤	0 ❌	0 🔥	2m 22s ⏱️

[Copilot]

Pull request overview

This PR updates the Python Foundry Hosted Agents Invocations hosting implementation to derive session partitioning from the Foundry request context (session/user IDs) to improve session isolation in hosted scenarios, and adds tests and sample documentation updates around invocations + streaming.

Changes:

• Add _partition_key() to compute a session cache key using Foundry request context (and fail fast when hosted protocol context is missing).
• Update _handle_invoke() to key the in-memory session cache by the partition key rather than request.state.session_id.
• Add unit + integration tests for invocations, and update the basic invocations sample README with a streaming curl example.

Reviewed changes

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

File	Description
python/samples/04-hosting/foundry-hosted-agents/invocations/01_basic/README.md	Documents a streaming invocation example alongside the existing non-streaming example.
python/packages/foundry_hosting/agent_framework_foundry_hosting/_invocations.py	Introduces partition-key session isolation based on Foundry request context; updates invocation handler behavior.
python/packages/foundry_hosting/tests/test_invocations.py	Adds unit tests covering partition-key behavior and invocation handler behavior (streaming/non-streaming).
python/packages/foundry_hosting/tests/test_invocations_int.py	Adds integration tests exercising the ASGI pipeline against a real Foundry endpoint (skipped unless env vars are provided).

[github-actions]

Automated Code Review

Reviewers: 5 | Confidence: 84% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

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Automated review by TaoChenOSU's agents

[github-actions]

Automated Code Review

Reviewers: 5 | Confidence: 88% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

---

Automated review by TaoChenOSU's agents

[moonbox3]

Following on the comment above about using a structured key to avoid collisions, should this sample use the same per-user session isolation as InvocationsHostServer? It currently looks up sessions using only the caller-provided agent_session_id, so another user who reuses that ID could access and modify the same AgentSession. Could the cache key include both the session ID and user ID from the request context?