[ext-agents]: evolve azure.ai.agent to service-level shape, retire agent.yaml, add migration

[huimiu]

Implements the agent-host and init parts of the unified azure.yaml design spec #8590 (docs/specs/unify-azure-yaml/spec.md) §1.1, §1.5, §2.3, §2.5, §2.7, §2.9.

Builds on #8675, which splits init output into per-resource sibling services but still writes the agent under config: and still emits agent.yaml / agent.manifest.yaml.

Summary

Lock the unified azure.yaml format for the agent host: move agent fields to service-level direct properties, drop the legacy agent files, and migrate older projects during a deprecation window.

Scope

• core schema (schemas/v1.0/azure.yaml.json):
  • Evolve the host: azure.ai.agent conditional from a $ref into config: to composing azure.ai.agent.json at the service level; keep project / runtime / docker / image, turn off config:.
  • Remove the superseded host: microsoft.foundry conditional and host example.
  • additionalProperties: true on the agent schema.
• init: write the agent service with direct service-level properties (no config: nesting) in both the manifest (init.go) and code-deploy (init_from_code.go) paths.
• Retire legacy files: stop emitting agent.yaml / agent.manifest.yaml for new projects (writeAgentDefinitionFile).
• Migration + telemetry: when agent.yaml / agent.manifest.yaml or the old config-nested host: azure.ai.agent shape is present, print a deprecation warning that links the migration guide, build the equivalent in-memory services so build/deploy still run, and emit a telemetry signal. Plan the rename/retire of CodeInvalidAgentManifest.
• Shared templating helper (§2.5): move the ${VAR} / ${{...}} expander (ExpandEnv) into a shared package the sibling Foundry extensions can reuse.

Out of scope (follow-ups)

• Per-resource real service targets in the sibling extensions → [ext-foundry]: make project/connection/toolbox Foundry resources real service targets #8774.
• $ref file includes / overlay overrides → [ext-foundry]: wire $ref file includes + overlay overrides for separate-services azure.yaml #8775.
• Built-in Bicep / provision-less flow (separate RFC).

Related

3-PR follow-up to #8590 / #8675:

• [ext-agents]: evolve azure.ai.agent to service-level shape, retire agent.yaml, add migration #8773 — agent shape + migration (this issue)
• [ext-foundry]: make project/connection/toolbox Foundry resources real service targets #8774 — per-resource service targets
• [ext-foundry]: wire $ref file includes + overlay overrides for separate-services azure.yaml #8775 — $ref file includes