diagram: Button silently drops unknown props (dead data-testid in ModuleDetails)

[bpowers]

Problem

The custom Button component (src/diagram/components/Button.tsx) destructures an explicit allowlist of props (variant, color, size, disabled, onClick, onMouseDown, className, style, startIcon, children, type, component, and three specific aria-* attributes) and does NOT spread the rest onto the rendered <button>. Any other prop is silently dropped.

Concretely, ModuleDetails.tsx (line ~288) passes data-testid="add-input-button" to Button, and that attribute never reaches the DOM:

• The data-testid is dead code.
• Any test written against it would silently query nothing. A test written during the diagram: readOnlyMode still permits most local model mutations #935 readOnlyMode capability-gate work had to fall back to matching the button's visible text instead.

TypeScript does not catch this because hyphenated JSX attributes (data-*, aria-*) are exempt from excess-property checking, so the API looks like it supports arbitrary DOM attributes but doesn't. Note the data-testid="model-ref-select" a few lines earlier in the same file works fine -- it's on a native <select> -- which makes the trap easy to fall into.

Why it matters

• Developer experience / test reliability: testids that appear to be wired up are no-ops, and getByTestId failures push tests toward brittle visible-text matching.
• API honesty: the component library reads as if it accepts standard DOM attributes; the silent drop violates that expectation with no compile-time or runtime signal.

Scope

Not Button-specific: only 6 of the 24 components in src/diagram/components/ spread rest props (Autocomplete, IconButton, icons, Snackbar, SvgIcon, TextField); the rest use explicit allowlists with the same silent-drop behavior. Button is the one with a concrete dead attribute today.

Possible approaches

1. Have Button (and, for consistency, the other allowlist components) forward rest props onto the underlying element -- at minimum data-*/aria-*, or extend React.ButtonHTMLAttributes<HTMLButtonElement> and spread {...rest} after the handled props.
2. Alternatively, remove the dead data-testid from ModuleDetails.tsx and document that the component library takes explicit props only.

Option 1 matches how the components are actually used and is the smaller ongoing-maintenance surface.

Context

Component: src/diagram (component library). Severity: low. Discovered during the readOnlyMode capability-gate work for #935 (branch production-risk-burndown).