Add Mermaid graph visualization

[FenjuFu]

What changed

• add a dependency-free Mermaid flowchart renderer to Graph.visualize()
• expose the same engine="mermaid" option through Application.visualize()
• preserve Graphviz as the default engine and reject Graphviz-only options on Mermaid
• support configurable Mermaid direction, condition labels, action state signatures, and external inputs
• escape user-controlled labels and use generated node IDs so arbitrary action names remain safe
• support writing Mermaid source directly to a UTF-8 file
• document the new API and add focused tests

Why

Graphviz image rendering requires both the Python package and a system dot executable. Mermaid source is useful in GitHub Markdown and other Mermaid-aware renderers without those dependencies.

This implements the text-generation portion of #414 without introducing Node, browser tooling, or a remote rendering service. Image rendering can remain a separate concern.

Validation

• 17 passed, 4 deselected across Graph, Mermaid, and visualization tests
• the four deselected tests are unchanged Graphviz image tests that require the unavailable local dot executable
• Black 23.11.0 (--line-length 100)
• isort 5.12.0
• Flake8 6.1.0
• ASF header check
• git diff --check

Contributed as a member of the iFLYTEK Astron community.

Closes #414