Every failed instrument build reports an extra "Cannot read properties of undefined (reading 'inputs')" error from the bundler plugin #1800

Every failed instrument build reports one error too many. The extra error is the bundler crashing on itself: Cannot read properties of undefined (reading 'inputs'). The plugin's onEnd hook reads result.metafile!.inputs unconditionally, but esbuild does not produce a metafile when the build has errors. So the hook throws a TypeError, and esbuild appends it to BuildFailure.errors as a second error attributed to instrument-bundler-plugin. Any author with a syntax error or a missing file then sees "Build failed with 2 errors" and an internal TypeError alongside their real mistake. This appears in the playground's error fallback (its "Cause" section shows the esbuild message), in serve-instrument's log, and in the API's instrument repo import log.

Where

packages/instrument-bundler/src/plugin.ts:86-95:

      build.onEnd((result) => {
        result.legacyScripts = [];
        Object.keys(result.metafile!.inputs).forEach((path) => {
          const script = legacyScripts.get(path.replace('bundle:', ''));
          // ...
        });
      });

Reproduce

1. bundle({ inputs: [{ name: 'index.ts', content: "export default { kind: 'FORM', ;" }] }).

Actual: the thrown error's cause.errors holds two messages: Expected identifier but found ";" and Cannot read properties of undefined (reading 'inputs') from instrument-bundler-plugin (verified on main). A missing import (import x from './nope.ts') also gets the same spurious second error.
Expected: cause.errors holds only the author's error.

Tests

packages/instrument-bundler/src/__tests__/build.test.ts: it('should report only the source error when compilation fails, so authors do not see an internal TypeError'). Run the real esbuild on a source with a syntax error and assert cause.errors has length 1.

e2e, in testing/src/specs/playground.spec.ts: introduce a syntax error in the editor and assert the error fallback does not contain reading 'inputs'.

Suggested fix

Return early from the onEnd callback when the build failed (if (!result.metafile) return;, or check result.errors.length).