Implementing trigger merge_request_code_conflict

What does this MR do and why?

Implementing trigger merge_request_conflict.

This is adding a new trigger type (Merge Request Code Conflict) that is triggered when open MRs have code conflict. They can trigger custom flows. See the related issue for more information.

Addressing: #592455 (closed)

Screenshots or screen recordings

How to set up and validate locally

• Enable the FF Feature.enable(:merge_request_code_conflict_flow_trigger)
• Create a flow and enable it on a project. See the reference project later in this MR. You can create your own Flow instead.
• Create an MR on a project
• Modify the project source code to cause a code conflict on the MR that you created before.
• You should see a flow started on the MR and some analysis posted on the MR. See the screenshot

Reference flow

version: "v1"
environment: ambient

flow:
  entry_point: "conflict_analyzer"

components:
  - name: "conflict_analyzer"
    type: AgentComponent
    prompt_id: "conflict_analysis_prompt"
    inputs:
      - from: "context:goal"
        as: "mr_iid"
      - from: "context:project_id"
        as: "project_id"
    toolset:
      - "get_merge_request"
      - "list_merge_request_diffs"
      - "list_commits"
      - "get_repository_file"
      - "list_repository_tree"
      - "create_merge_request_note"
    ui_log_events:
      - "on_agent_final_answer"
      - "on_tool_execution_success"
      - "on_tool_execution_failed"

routers:
  - from: "conflict_analyzer"
    to: "end"

prompts:
  - name: "MR Conflict Analyzer"
    prompt_id: "conflict_analysis_prompt"
    unit_primitives: []
    prompt_template:
      system: |
        You are an expert code conflict analyst. Your job is to analyze a GitLab merge request
        that has a merge conflict and post a clear, actionable comment explaining why the
        conflict occurred and how to resolve it.

        Steps:
        1. Use get_merge_request with project_id={{project_id}} and mr_iid={{mr_iid}}
        2. Use list_merge_request_diffs to get the full diff
        3. Use list_commits to understand recent changes on both branches
        4. Use get_repository_file on conflicting files if more context is needed
        5. Post a comment using create_merge_request_note with the analysis below

        The comment MUST follow this structure:

        ## 🔀 Merge Conflict Analysis

        ### Conflicting Files
        List each file and what changed in both branches.

        ### Root Cause
        Explain clearly why the conflict occurred.

        ### Resolution Steps
        Numbered steps to resolve locally:
        - git checkout <source-branch>
        - git fetch origin
        - git merge origin/<target-branch>
        - For each conflicting file: what to keep, remove, or combine
        - git add <files>
        - git commit && git push

        ### Tips
        Any extra context (e.g. which side should take precedence, or if rebase is preferred).

      user: |
        Project ID: {{project_id}}
        MR IID: {{mr_iid}}

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #592455 (closed)