Add optional Mermaid flow-graph visualization to the PR comment

[sfc-gh-pvillard]

Summary

Adds an optional per-process-group Mermaid flow-graph to the PR comment, giving reviewers a visual of the structural changes alongside the existing textual description.

• New flow-graph action input (default false — no change to existing behaviour when unset).
• When enabled, each process group with structural changes (components/connections added or removed, or a connection rerouted) gets a collapsible flowchart overlaying the diff: 🟩 added / 🟥 removed / 🟧 modified / ⬜ unchanged. Node shapes distinguish processor (rectangle), port (rounded), funnel (circle); a processor's type shows as <Type> and a child-group port is tagged [group name].
• Config-only groups keep the text description with no graph; a group whose graph would exceed a legibility/size limit falls back to text.
• Graph logic lives in a new com.snowflake.openflow.graph.FlowGraph class, mirroring the checkstyle package layout — FlowDiff only wires it in.
• Also makes the diff bullet ordering deterministic (content-based instead of the JVM identity hashCode), so the comment no longer reorders on unrelated code changes.

Example

flowchart TB
classDef added fill:#d1fae5,stroke:#16a34a,color:#166534;
classDef removed fill:#fee2e2,stroke:#dc2626,color:#991b1b,stroke-dasharray: 5 5;
classDef modified fill:#fef3c7,stroke:#d97706,color:#92400e;
classDef context fill:#e5e7eb,stroke:#6b7280,color:#374151;
n0["Generate<br>&#60;GenerateFlowFile&#62;"]:::modified
n1(["Ingest In"]):::added
n2["Old Sink"]:::removed
n0 -->|"success"| n1
n0 -.->|"success"| n2

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Test plan

• mvn clean package — build passes, 53 tests (added FlowGraphTest + 4 fixtures covering cross-group ports, funnel, reroute, oversize suppression, escaping, and config-only groups).
• Verified on a real production connector PR: graph-enabled output renders correctly; graph-disabled output is unchanged in content.
• Reviewer: sanity-check the rendered graph on a live PR comment.