Python: Foundry hosting: harden Responses integrations and isolation samples #8746

Part of #8742. This is a Responses agent integrations and isolation implementation slice—not a samples-only cleanup. Its PR closes this child issue and depends on the scope/Responses-agent work in #8743 and #8744.

Deliverable

• Make the session-file example read only from a dedicated directory under the current hosted sandbox's $HOME; reject traversal, file/directory symlinks and oversized reads. Demonstrate upload and cross-sandbox file isolation.
• Keep custom MAF session storage scoped by trusted user and hosted sandbox. Use managed identity for the Cosmos example rather than copying a connection string into the agent; enforce conditional updates so concurrent turns cannot silently overwrite state.
• Construct the Foundry Memory integration from the trusted per-request user scope rather than passing a literal template string as the scope. Document intentional per-user memory sharing across that user's sessions, and the project-scoped RBAC it needs.
• Wire request-owned Toolbox/skills, Azure AI Search RAG, Monty CodeAct, observability, and the hosted harness examples to the agent lifecycle introduced in the preceding PRs. Keep sample-specific external tools and storage explicitly scoped; do not pass a previous caller's platform call ID to a new request.

Review and validation

Ship each directly related sample source and README with its implementation. Add isolated file/Cosmos/Memory negative tests, plus targeted hosted smoke checks where credentials/resources exist. State external prerequisites plainly: GitHub MCP PAT and Telegram/Key Vault require separate configuration; Hyperlight needs KVM unavailable in the default Foundry hosted runtime. Do not claim those credential- or hypervisor-gated behaviors were exercised. Avoid changing the native workflow APIs in this slice.

Related narrower issues: #6558 (session docs), #7690 (Toolbox call ID), and #7916 (citations); do not automatically close them without verifying their full acceptance criteria. The PR should say Part of microsoft/agent-framework#8742; merge all slices before one beta release.