.NET: Preserve hosted session key boundaries

[Ricky-7-Yan]

Motivation & Context

Hosted session stores must preserve the boundaries between agent, user, and conversation identifiers. Delimiter-based string keys allow distinct identifier tuples containing : to collapse to the same cache entry, which can mix session state across requests.

Description & Review Guide

• What are the major changes? Python Invocations hosting now serializes hosted session/user identifiers as a compact JSON array, including the standalone break-glass example. The .NET Foundry in-memory store now uses a strongly typed composite key. Focused regression tests cover distinct colliding tuples and repeated-request continuity.
• What is the impact of these changes? Distinct hosted identifier combinations remain isolated while existing local-mode keys and same-tuple session reuse stay unchanged.
• What do you want reviewers to focus on? Please review the hosted key representation and the compatibility of null/empty agent and user components in the .NET store.

Related Issue

Fixes #8251

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]

🟢 Approval recommended

The implementations preserve identifier boundaries and existing null, empty, and local-mode behavior with focused regression coverage.

Pull request overview

Preserves hosted session isolation by replacing ambiguous delimiter-based keys with boundary-safe representations.

Changes:

• Serializes Python hosted identifiers as compact JSON arrays.
• Uses a typed composite key for the .NET in-memory store.
• Adds regression tests for collision isolation and session continuity.

File summaries

File	Description
python/samples/04-hosting/foundry-hosted-agents/invocations/break_glass/main.py	Updates the sample’s hosted session key format.
python/packages/foundry_hosting/tests/test_invocations.py	Tests collision isolation and repeated-session reuse.
python/packages/foundry_hosting/agent_framework_foundry_hosting/_invocations.py	Generates unambiguous hosted partition keys.
dotnet/tests/Microsoft.Agents.AI.Foundry.Hosting.UnitTests/InMemoryAgentSessionStoreTests.cs	Verifies composite identifier boundaries.
dotnet/src/Microsoft.Agents.AI.Foundry.Hosting/InMemoryAgentSessionStore.cs	Replaces concatenated keys with a typed record key.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Balanced

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[Vincent Biret (baywet)]

Thanks for the contribution!

(to other reviewers, I've only reviewed the dotnet side of things)