Show actionable next-step hints when `azd ai agent init` fails

[spboyer]

Problem

When \�zd ai agent init\ fails in non-interactive mode, error messages are raw and don't suggest fixes. Users must discover workarounds through trial and error.

Examples of unhelpful messages:

• 
  o default response for prompt 'Enter a unique environment name:'\ -- should suggest -e \
• \subscription selection required but cannot prompt in --no-prompt mode\ -- should suggest \�zd config set defaults.subscription \
• \cannot prompt for deployment configuration in non-interactive mode\ -- should suggest the flag from azd ai agent init: Add --model-deployment flag for non-interactive usage #6816

Proposed Solution

Apply the hint pattern established in core azd (PRs #6801-6804, #6810) to extension error messages:

• Detect specific error conditions
• Display actionable \Hint:\ messages with exact commands to run
• Example: \Hint: Set a default subscription with 'azd config set defaults.subscription '\

Related Issues

• azd ai agent init: Add --model-deployment flag for non-interactive usage #6816 - Model deployment flag
• azd ai agent init: Default environment name to directory name #6817 - Environment name default
• azd ai agent init: Auto-select subscription in non-interactive mode #6818 - Subscription fallback
• Allow azd ai agent init to run non-interactively in CI and AI assistants #6819 - Documentation

[spboyer]

Reviewed during issue triage — keeping open.

Partial progress: the --model-deployment no-prompt error path already emits a hint ("use --model-deployment to specify a deployment name in non-interactive mode"). Core azd also now has the ErrorWithSuggestion framework from PR #6827 plus the YAML-driven error_suggestions.yaml pipeline.

TL;DR remaining work:

• Audit error sites in cli/azd/extensions/azure.ai.agents/internal/cmd/ and wrap user-fixable failures in ErrorWithSuggestion (populate Err, Suggestion, Message, Links — see cli/azd/AGENTS.md for the completeness rule).
• Specific gaps to cover:
  • Missing environment name → suggest -e <name>.
  • Missing subscription in --no-prompt → suggest azd config set defaults.subscription <id>.
  • Generic Foundry resolution failures → include the --project-endpoint / --subscription hint (matches the fix in Bugfix: Honor --subscription and --project-endpoint flags over stored env values #8093).

[therealjohn]

Related #7975