Expand native Mermaid flowchart syntax support (#48895)

## What changed

- Default `flowchart` and `graph` to top-down layout when no direction is specified.
- Support solid and dashed edges with directed, undirected, or bidirectional endpoints: `-->`, `---`, `<-->`, `-.->`, `-.-`, and `<-.->`.
- Accept pipe labels on these edges and spaced directed infix labels such as `A -- send --> B` and `A -. retry .-> B`.
- Expand `&` groups into all source-to-target edges, including in chains. For example, `A & B --> C & D` creates four edges. Enforce the 24-edge limit after expansion and cap each group at 24 node references.

## Testing

Add coverage for equivalent syntax, edge styles and endpoints, malformed input, and expansion limits. Extend rendering snapshots across all four directions and the TUI native-renderer test with the new syntax.

GitOrigin-RevId: 6e51653ebf6b8b7e7e1edec0c2e09a5d9c8dec74

codex-rs/tui/src/markdown_render/mermaid_tests.rs

@@ -21,6 +21,7 @@ fn mermaid_fences_use_native_renderer_for_every_family() {
     for source in [
         "%% heading\nflowchart TD; A --> B",
         "graph LR; A --> B",
+        "flowchart; A -- send --> B & C; B <-.-> C",
         "sequenceDiagram; A->>B: request; B-->>A: response",
         "stateDiagram-v2; [*] --> Active; Active --> [*]",
         "stateDiagram; [*] --> Active; Active --> [*]",