fix(render): surface an error for unhandled renderer tags instead of "[object Object]"

[sagarswamirao]

What

When a field carries a renderer tag that resolves to a renderer name but no plugin handles it, the renderer rendered [object Object] (or, once an error was thrown, an empty red error box). This makes both cases surface a clear, inline error message instead.

Why

big_value supports child-only config keys (e.g. sparkline, comparison_field) that are only valid on a basic field inside an activated big_value. If a user instead puts a child-only key on the view itself -- for example:

# big_value { sparkline=monthly_traffic }
view: summary is { ... }

the big_value plugin declines to match (child-only config, no activating big_value on the parent), so no plugin attaches to the field. The Malloy hover/lint already flags this as invalid. But at render time:

1. shouldRenderAs still maps the big_value tag to renderAs = 'big_value'.
2. applyRenderer finds no plugin and no switch case for 'big_value', so it hit the default branch.
3. The default did String(dataColumn.value). For a structured root cell (a record/array), that produced [object Object] filling the entire result panel.

So the hover lint said "this is wrong" but Run SQL rendered garbage. The two should agree -- a misconfigured tag should fail with a visible, actionable error in both places.

While wiring that up, a second, pre-existing bug surfaced: throwing an error produced an empty red box rather than the message. Solid's ErrorBoundary passes the thrown error as the first fallback argument, but the fallback read errorProps.error?.message and otherwise fell through to the error object itself, which renders as nothing when used as a JSX child. That affected all render-time error display, not just this case.

Changes

• apply-renderer.tsx -- in the default case, render scalar values via String() as before, but throw a descriptive error for structured (object/array) values instead of emitting [object Object].
• render.tsx -- resolve the ErrorBoundary message from the {error} wrapper, a thrown Error, or a raw string, coercing to a string so the message is always displayed (fixes the empty error box).

Compatibility

Backward compatible. Scalar values still render exactly as before; only the previously-broken [object Object] path now surfaces a visible error, and existing error messages now display instead of an empty box.

Test

jest --selectProjects malloy-render -- 92 passed. Verified end-to-end in the VS Code extension: the misconfigured # big_value { sparkline=... } view now shows Malloy render: no renderer available for 'big_value' on field 'root'. Check the 'big_value' tag -- its configuration may be incomplete or unsupported. instead of [object Object].