Python: Foundry hosting: add parsed and durable Invocations agent runs #8745

Part of #8742. This issue tracks the Invocations agent implementation PR, to be closed by that PR rather than by the umbrella. It builds on the trusted state/scope foundation in #8743; native workflows belong to a later sub-issue.

Deliverable

• Accept application-defined JSON through an optional parse_request callback returning MAF messages, runtime options and streaming intent. Give developers a prepare_options hook and a configurable unsupported-options policy without mutating agent defaults.
• Persist MAF AgentSession across hosted process replacement under trusted platform user+sandbox scope. Serialize turns in-process and fail explicitly on cross-process CAS conflicts; avoid stale process-only session caches.
• Return JSON for non-streaming requests and framed SSE delta/done/error for streaming. Finalize streams with the public API supported by the released agent-framework-core dependency; preserve partial session state on interrupted streams where safe.

Explicit compatibility choice

The new JSON/SSE wire contract is the default. Provide a developer opt-in, once-warned deprecated mode for the previous plain-text response/raw text-chunk behavior. No caller should receive the old format unexpectedly. Document when that opt-in can be removed; do not silently switch a deployed caller's wire format.

Review and validation

Include the basic Invocations agent/parser example and its README in this PR. Test two-turn session continuity across host restarts, option allow/deny hooks, stream completion and failure framing, same-user/different-sandbox isolation, and the deprecated wire mode. Exercise a deployed Invocations endpoint where project credentials are available. Do not add native workflow= handling or its typed workflow sample here.

The PR should use Closes <this child issue> and Part of microsoft/agent-framework#8742; all redesign PRs land before one beta release.