[WIP][not-for-review] azure.ai.agents: revamp next-step + doctor for unified azure.yaml

[antriksh30]

Summary

[WIP] [not-for-review] — This PR is not ready for review. It is staged ahead of the
unified-azure.yaml core changes and is intended to be finalized once that work lands.

Reworks the two azure.ai.agents extension features I previously shipped —
next-step suggestions (internal/cmd/nextstep/) and azd ai agent doctor
(internal/cmd/doctor/) — to source all state from the unified azure.yaml design
instead of the legacy three-file layout.

• Before: azure.yaml (host: azure.ai.agent + a config: block) + agent.yaml
  (runtime def) + agent.manifest.yaml (templated resources).
• After: a single azure.yaml service entry, host: microsoft.foundry, with
  deployments / connections / toolboxes / skills / routines / agents as
  top-level service properties (no config: indirection); per-agent runtime fields
  (project, docker, protocols, env, …) nest under agents[].

Design references: the product brief (preview repo simple/complex branches) and the
engineering spec #8590 (docs/specs/unify-azure-yaml/spec.md).

⚠️ Dependency / status

This change is gated on a separate workstream (core azure.yaml JSON-schema support for
host: microsoft.foundry, the new service-target registration, and the init rework). Until
that lands, these two features will be no-ops for existing (old-layout) projects — which
is expected and accepted. The code is structured so field-shape true-ups against the final
implementation are localized to one decode seam.

What changed

nextstep

• New nextstep/foundry_config.go — the single decode seam. Decodes
  ServiceConfig.AdditionalProperties (forwarded by core over gRPC, no env-subst) into a
  local read-model, and resolves data-side $ref: file includes with the spec's
  overlay overrides (sibling keys win) and project: path rebasing (relative to the
  referenced file). ${VAR} / ${{...}} are preserved verbatim.
• state.go — collectFoundry produces one ServiceState per agent (with Kind +
  owning ServiceName) plus aggregated model/toolbox/connection refs; host const →
  microsoft.foundry; an explicit endpoint: marks the project as present.
• Deleted manifest.go (the agent.manifest.yaml walker) and dropped {{ }}
  placeholder handling entirely; missing-var detection now scans agents[].env.
• resolver.go — removed the placeholder branch (switch 4 → 3 arms).

doctor

• Host const → microsoft.foundry; removed the now-obsolete local.agent-yaml-valid
  check (structural validity is covered by local.azure-yaml); repointed the
  manual-env-vars skip-cascade to local.agent-service-detected.
• Updated all user-facing strings/docs from agent.yaml / agent.manifest.yaml /
  azure.ai.agent → azure.yaml / microsoft.foundry.

tests

• Rewrote the affected tests against in-memory AdditionalProperties fixtures (preserving
  OpenAPI / Bicep-classification coverage); added decode-seam, protocol, env-ref, and
  resource-aggregation tests.

Scope

Touches only internal/cmd/nextstep/ and internal/cmd/doctor/ (+ their tests). Does
not touch init.go, listen.go, or service_target_agent.go — those belong to the
unification workstream.

Verification

• go build ./... ✅
• go test ./internal/... ✅ (whole extension)
• golangci-lint run (nextstep + doctor) ✅ 0 issues
• cspell ✅ · gofmt -l clean · copyright headers present

Known true-ups (tracked in code comments)

• isDeployed assumes a per-agent AGENT_<KEY>_VERSION env var — verify against the new
  service target's actual convention (TODO(unify true-up) in state.go).
• Nested-$ref / instructions path rebasing intentionally out of scope (not consumed by
  nextstep).