Remove redundant role assignments for hosted agents (service now auto-assigns)

[v1212]

Summary

The Foundry service now automatically assigns required roles to per-agent managed identities on agent creation. The azd agents extension should remove its client-side role assignment logic, which is now redundant.

Background

The VS Code extension (Skylight) has already removed this logic:

• https://github.com/microsoft/Skylight/pull/4754 — Clean up hosted agent role assignments (simplified to Foundry User only)
• https://github.com/microsoft/Skylight/pull/4910 — Drop client-side per-agent identity role assignment entirely

Changes needed in azd

File	Current behavior	Action
internal/project/agent_identity_rbac.go	Postdeploy assigns "Azure AI User" to each agent MI	Remove (or convert to verify-only)
internal/project/developer_rbac_check.go	Predeploy checks developer has roleAssignments/write	Remove this specific check
internal/cmd/listen.go	Postdeploy calls EnsureAgentIdentityRBAC()	Remove call
resources/scaffold/base/modules/ai-search-conn.bicep	Assigns Search roles to Project MI	Remove role assignment resources
internal/cmd/doctor/checks_agent_identity_roles.go	Diagnoses agent MI role assignments	Remove or make info-only

Impact

• Faster deploy (no RBAC polling/propagation wait)
• Lower permission requirements for developers (no longer need roleAssignments/write)
• Fewer transient failures from RBAC propagation delays