[ext-agents] Add microsoft.foundry service target (foundation: single hosted agent)

[huimiu]

Summary

Wire the new host: microsoft.foundry service target in the azure.ai.agents extension, implementing the foundation of design spec #8590 §2.1, §2.2, and §2.6.

A single microsoft.foundry service represents a whole Foundry project, with agents declared inline as top-level service properties delivered on ServiceConfig.AdditionalProperties (not under config:).

Scope (foundation)

• Register the microsoft.foundry service target (listen.go + extension.yaml).
• Bind AdditionalProperties → typed FoundryProjectConfig / FoundryAgent (aligned with Agent.json / microsoft.foundry.json from feat(agents): add microsoft.foundry azure.yaml schema #8603).
• Initialize validation: single hosted agent, exactly one deploy mode, project resolution, shared auth setup.
• Endpoints and GetTargetResource.
• Package/Publish/Deploy for a single hosted agent end-to-end:
  • prebuilt image: → CreateAgentVersion
  • code-deploy runtime: (python/dotnet) → ZIP package + create/update agent version

Out of scope (follow-ups)

• Multi-agent fan-out, bounded concurrency, per-agent error attribution.
• Container (docker:) builds for the new host (core does not drive a per-agent container build yet).
• Data-plane reconcile: deployments, connections, toolboxes, skills, routines (§2.8).
• $ref include resolution wiring (Resolve $ref file includes with overlay overrides in Foundry config #8626/feat(agents): resolve $ref file includes with overlay overrides in Foundry config #8627) and instructions prompt-file loading.
• init rework and old-file deprecation.

Notes / open items

• Code-deploy entry point is derived from startupCommand (the agent schema models entry via startupCommand, not an explicit field); this is the documented seam if the schema later adds one.
• Endpoint-only projects still require AZURE_AI_PROJECT_ID for GetTargetResource, consistent with the azure.ai.agent host.