Improve Service Provider error handling

[maximiliantech]

Goal

Provide a clear and actionable error handling experience for Service Provider reconcilers, so users and operators can understand and resolve issues without diving into logs or source code.

Problem Statement

Service Provider reconcilers currently lack consistent error handling:

• Missing or unclear status conditions when prerequisites are not met (e.g. referencing a non-existent ManagedControlPlaneV2)
• No standardized error condition patterns across Service Providers
• Users often can't tell what went wrong or how to fix it from the resource status alone

Possible more sub-issues

• Error handling in Service Providers if requested version is not found in available version list of ProviderConfig
• Update service provider development guide about status handling
• End-user documentation: What are common pitfalls when requesting new services? How to read the status conditions correctly?

--> Better prepare this issue and define more sub-issues out of the list above ⬆️

Acceptance Criteria

• Define error condition conventions for Service Provider reconcilers
• Implement clear error conditions for common failure modes (missing references, invalid config, dependency not ready)
• Service Provider template reflects the agreed error handling patterns
• Existing Service Providers adopt the conventions

Out of Scope

tbd