Support Conditional Step Execution with "when" Expressions

[shalev007]

Problem:
Right now, every step in a RestBook playbook runs unconditionally. For real-world use cases (like syncing users or skipping optional steps), we need a way to conditionally execute steps based on variable values or previous responses.

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✅ What to Implement

• Add a new when: field to StepConfig
• Allow a Jinja2 expression to be evaluated at runtime
• If when: evaluates to false, the step is skipped
• Log skipped steps clearly (e.g., "Step 2 skipped (when=false)")
• Skipped steps should not store or mutate variables

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🔧 Example Syntax

steps:
  - name: check-user
    request:
      method: GET
      endpoint: /users/{{ user_id }}
    store:
      - var: user_exists
        jq: '.id != null'

  - name: create-user
    when: "{{ not user_exists }}"
    request:
      method: POST
      endpoint: /users
      data:
        id: "{{ user_id }}"
        email: "{{ user_email }}"

💡 Why It Matters

• Enables idempotent API workflows by skipping redundant operations
• Reduces unnecessary API calls and avoids unintended side effects
• Makes playbooks more dynamic, readable, and declarative
• Helps backend/data engineers build safer integrations with fewer steps
• Opens the door for richer control flow (like branching logic)

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📎 Related Ideas

• Allow when: at the phase level to skip entire phases conditionally
• Log skipped steps clearly in both output and summary logs
• Add support for --show-skipped in CLI to report skipped steps explicitly
• Future: Add else: or otherwise: clause to support step-level fallback
• Combine with dry-run mode to preview which steps would run or skip