Remove dead COBO code from azure.ai.agents extension; guard postdeploy handler with ShouldRun

[Copilot]

The postdeployHandler in the azure.ai.agents extension called EnsureAgentIdentityRBAC() without checking whether the project actually uses agents. This caused AZURE_AI_PROJECT_ID not set errors in projects that have the extension installed but no agent services. Separately, the entire COBO flow (~1130 lines) was dead code that no longer functions.

Bug fix (#7344)

Added a ShouldRun guard in postdeployHandler so EnsureAgentIdentityRBAC is skipped for non-agent projects:

func postdeployHandler(ctx context.Context, projectParser *project.FoundryParser, args *azdext.ProjectEventArgs) error {
	shouldRun, err := project.ShouldRun(ctx, args.Project)
	if err != nil {
		return fmt.Errorf("failed to determine if extension should run: %w", err)
	}
	if !shouldRun {
		return nil
	}
	// ...
}

Dead code removal (#7201)

• parser.go: Removed SetIdentity, CoboPostDeploy, all COBO types (AgentRegistrationPayload, AgentDefinition, etc.), and 12+ helper functions (assignAzureAIRole, registerAgent, deactivateHelloWorldRevision, testDataPlane, etc.). File went from 1197 → 66 lines.
• listen.go: Removed SetIdentity() calls from preprovisionHandler/predeployHandler, simplified their signatures. Replaced CoboPostDeploy + EnsureAgentIdentityRBAC with just the latter behind ShouldRun.
• go.mod: Dropped unused deps armappcontainers and armresources.
• Exported ShouldRun (was shouldRun) so the cmd package can call it directly.