Add pre-tool-validation lifecycle event for extensions

[spboyer]

Extensions cannot provide guidance or suggestions when required tool validation fails because lifecycle events fire after EnsureAllTools() completes successfully. This creates a catch-22 where extensions that want to suggest alternatives (like remote build when Docker is unavailable) never get the opportunity to run.

Current Execution Order

1. Initialize
2. EnsureAllTools() ← Fails if Docker daemon not running
3. [COMMAND EXITS WITH ERROR]
4. Lifecycle events (prepackage, preprovision, predeploy) ← Never reached

Example Use Case

The azd-ext-doctor extension aims to suggest enabling remote build when Docker daemon is unavailable:

func onPrePackage(ctx context.Context, args *azdext.ServiceEventArgs) error {
    // Check if Docker required but not running
    // Suggest: azd doctor configure remote-build
    // But this never executes because EnsureAllTools fails first
}

Current behavior: User runs azd package with Docker daemon stopped → immediate error
Desired behavior: Extension suggests remote build alternative before failing

Proposed Solution

Add a new lifecycle event that fires before tool validation, allowing extensions to:

• Inspect required tools
• Provide suggestions for missing/unavailable tools
• Offer configuration alternatives
• Guide users through remediation

Suggested Event Names (choose one)

• pretoolcheck - Fires before EnsureAllTools()
• preinit - Fires at the very beginning of command execution
• toolcheck - Fires with tool validation context

Example Extension Usage

host.WithProjectEventHandler("pretoolcheck", func(ctx context.Context, args *azdext.ProjectEventArgs) error {
    // Extension can inspect required tools
    requiredTools := args.RequiredTools
    
    for _, tool := range requiredTools {
        if tool.Name == "docker" && !isDockerDaemonRunning() {
            fmt.Fprintln(os.Stderr, "💡 Tip: Enable remote build to build without local Docker")
            fmt.Fprintln(os.Stderr, "Run: azd doctor configure remote-build")
        }
    }
    
    return nil
})

Modified Execution Order

1. Initialize
2. → NEW: Fire pretoolcheck lifecycle event ← Extensions can provide guidance
3. EnsureAllTools() ← Fails if tools missing/unavailable
4. Lifecycle events (prepackage, preprovision, predeploy)

Benefits

1. Better UX: Users get actionable suggestions before seeing cryptic tool errors
2. Extension Capabilities: Extensions can provide domain-specific guidance
3. Graceful Degradation: Extensions can suggest workarounds (remote build, alternative tools, etc.)
4. Proactive Help: Catch tool issues early with helpful context

Related Code Locations

• cmd/package.go#L134 - Package command calls EnsureAllTools
• cmd/provision.go#L197 - Provision command calls EnsureAllTools
• cmd/deploy.go#L217 - Deploy command calls EnsureServiceTargetTools
• pkg/tools/ensure.go#L39-L87 - Tool validation logic

Alternative Considered

Extensions could use standalone commands (like azd doctor check), but this:

• Requires users to know about the extension
• Adds extra manual steps
• Doesn't integrate with normal azd up/package/deploy workflows
• Loses context about which command the user actually wanted to run

Implementation Notes

The event should provide:

• List of required tools for the operation
• Service/project context
• Ability to output to stderr without disrupting normal flow
• Should NOT block tool validation (run even if extension errors)

Real-World Impact

The azd-ext-doctor extension (https://github.com/spboyer/azd-ext-doctor) would immediately benefit from this feature, providing users with actionable suggestions when tools are unavailable, improving the developer experience significantly.

[spboyer]

We could also propose moving these tool checks to the doctor extension for management and agility.