Validate released flows have at least 1 step

What does this MR do and why?

We need to ensure that released flows have at least 1 step defined.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the global_ai_catalog feature flag
2. Create a flow without any steps, which has not yet been released:

mutation {
  aiCatalogFlowCreate(
    input: {name: "Test flow", description: "Test flow", projectId: "gid://gitlab/Project/1000000", public: true, steps: []}
  ) {
    errors
    item {
      id
    }
  }
}

3. Update the flow to release it, and observe that there will be an error:

mutation {
  aiCatalogFlowUpdate(input: {
    id: "gid://gitlab/Ai::Catalog::Item/<REPLACE_WITH_ID_OF_FLOW>"
    release: true
  }) {
    errors
  }
}

4. Create an agent

mutation {
  aiCatalogAgentCreate(
    input: {name: "Name", description: "An agent", projectId: "gid://gitlab/Project/1000000", systemPrompt: "sys", userPrompt: "user",  public: true, release: true}
  ) {
    errors
    item {
      id
    }
  }
}

5. Attempt to release the version with steps defined:

mutation {
  aiCatalogFlowUpdate(input: {
    id: "gid://gitlab/Ai::Catalog::Item/<REPLACE_WITH_ID_OF_FLOW>"
    release: true
    steps: [{ agentId:   "gid://gitlab/Ai::Catalog::Item/<REPACE_WITH_ID_OF_AGENT>" }]
  }) {
    errors
  }
}

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Related to #565649 (closed)