Migrate bundled and legacy toolbox diagnostics

[huimiu]

Parent issue: #8710

Why this is needed

The split toolbox diagnostics are being handled separately, but #8710 also needs unified azure.yaml diagnostics to preserve the behavior of bundled and legacy manifest toolboxes. Without this follow-up, Next-step suggestions and Doctor results can still give incomplete or outdated remediation for those configuration sources.

Scope

• Recognize bundled toolboxes declared in unified azure.yaml service configuration.
• Preserve legacy agent.manifest.yaml toolbox compatibility while unified configuration is adopted.
• Surface toolbox configuration and referenced-file loading errors instead of silently falling back.
• Provide source-aware Next-step and Doctor remediation for missing endpoints and migration-required configurations.
• Add regression coverage for bundled, legacy, mixed-source, empty-declaration, and configuration-load-error cases.

Out of scope

• Split toolbox service discovery and endpoint probing covered by the first toolbox slice of Migrate next step suggestions and doctor command to make it work with unified azure.yaml #8710.
• Model and connection migration.
• Changes to toolbox provisioning or deployment runtime behavior.

Acceptance criteria

• Bundled and legacy toolbox resources are represented consistently in assembled state.
• A failed unified configuration or $ref load is visible in Doctor and does not trigger an incorrect manifest fallback.
• Next-step and Doctor guidance identifies the correct remediation for each toolbox source.
• Existing legacy projects remain supported without regressions.