Fix mermaid rendering for debug plans already on disk

[Nathan (nturinski)]

Stacked on #1813 — please review/merge that one first. Base branch is nturinski-mermaid-label-quoting-490.

Why a second PR

#1813 fixes the generation side: it adds an unconditional "quote every node and edge label" rule to resources/agents/azure-debug-plan/references/plan-template.md and to the CoR docs instructions. That is prevention at the source, and it has two gaps:

1. It does nothing for a .azure/vscode-debug-plan.md that is already on disk in a user's workspace. Those diagrams still fail to render today.
2. A template rule is a request the model can decline. Normalization is enforced.

This PR closes both gaps by normalizing at render time.

The failure

The generated diagram contains:

API -->|@azure/storage-blob| AZ

Mermaid v11 reserves @ for edge-ID (e1@-->) and node-metadata (id@{ shape: ... }) syntax, so an @ at the start of an unquoted label lexes as LINK_ID and the whole diagram fails to parse — the webview shows mermaid's "Syntax error in text" bomb graphic instead of the architecture diagram. An @ mid-label is fine; quoting the label fixes it.

This is not a mermaid version regression — it reproduces identically on 11.14.0, 11.15.0 and 11.17.0, so the mermaid dependency is untouched.

parseLocalDebugPlanMarkdown is innocent: it extracts the fenced block faithfully. The gap was that MermaidBlock handed that string straight to mermaid.render() with no normalization.

What changed

• New src/webviews/copilotOnRails/views/utils/quoteMermaidLabels.ts — quotes any flowchart node or edge label that is not quoted already.
• LocalPlanView.tsx's MermaidBlock now calls mermaid.render(id, quoteMermaidLabels(code)). The error fallback still shows the code the author actually wrote.
• New test/copilotOnRails/quoteMermaidLabels.test.ts — 22 unit tests in the existing suite's style, no new test framework.

The transform is deliberately narrow, because the failure mode of a bad rewrite is a diagram that used to work and no longer does:

Guard	Why
Only graph / flowchart diagrams are rewritten	[], {} and | mean something else entirely in sequence, class, ER, state and pie diagrams — those come back byte-identical
Lines containing @{ are exempt from the rhombus rule	v11 node/edge metadata (A@{ shape: rect }, e1@{ animate: true }) is brace-delimited configuration, not a label. Rewriting e1@{ animate: true } to e1@{"animate: true"} would change what it means. This is explicit rather than accidental
Rewrites skip the inside of existing "…" strings	Keeps an already-quoted label intact even when it contains brackets or braces (A["Blob (hot tier) [preview]"]), and makes the transform idempotent
Cylinder/circle/stadium/subroutine/hexagon delimiters matched before the plain […] rule; / and \ (parallelogram/trapezoid) left alone	Shapes are preserved rather than flattened to a rectangle
Comment lines and leading YAML frontmatter pass through	They are not diagram body

Net effect: it is a no-op on any diagram that already renders.

Validation

Real mermaid 11.17.0 under jsdom, driving the actual parseLocalDebugPlanMarkdown over plan markdown — 107/107 checks pass:

• The reported failing diagram: fails as-authored, parses after normalization.
• All four checked-in plan fixtures parse before and after, and normalization is idempotent on each:
  • test/testProjects/copilotOnRails/attendance/vscode-debug-plan.md
  • test/testProjects/copilotOnRails/scrapbook/vscode-debug-plan.md
  • evals/grader-certification/reference-node-fullstack/.azure/vscode-debug-plan.md
  • evals/grader-certification/stage-local-dev/.azure/vscode-debug-plan.md
• Valid before and after, uncorrupted: already-quoted labels, subgraph, classDef + class, bare A --> B --> C, rhombus, circle, cylinder, hexagon, stadium, subroutine, parallelogram, trapezoid, style, linkStyle, click callbacks, markdown-string labels (["`**bold**`"]), %% comments, %%{init: …}%% directives, YAML frontmatter, and v11 id@{ shape } / e1@--> metadata.
• Non-flowchart diagrams and v11 metadata lines verified byte-identical.
• @-leading labels broken before / fixed after in every shape: edge label, rect, cylinder, circle, rhombus.

Repo gates — all green:

Gate	Result
npm run build:check (tsc --noEmit)	✅
npm run lint	✅
evals: drift	✅ 17 agent contracts intact — no agent asset changed, so agent-assets.lock.json is untouched
evals: typecheck, lint, certify (152/152), imports:self-test, stacks:check, gates, phases:check, seed:contract, clean-machine:check	✅

evals/results/grader-certification/offline/report.json was restored after certify so its generatedAt churn does not leak into this PR, and package-lock.json was restored after the local npm i. The diff is three files.

Docs

No update needed in docs/copilot-create-project.md: no command, MCP tool, agent, .azure/* artifact or workspaceState key changed, and no UI surface was added, removed or restyled. The debug plan view's intended appearance is unchanged — this only makes diagrams that were showing the error graphic render as designed. 08-debug-plan-view.png is therefore only stale if it happens to have been captured while a diagram was failing to parse; worth a glance, but no placeholder was added or removed.