SVG fixtures are loaded more leniently in tests than in the webview #76

The problem

The SVG fixtures are parsed one way by the tests and another way by the code they stand in for.

• Tests: packages/extension/test/svg-theme.test.ts:39 does document.body.innerHTML = text.slice(start) — the HTML parser, which is deliberately forgiving and never reports an error.
• The webview: packages/extension/src/webview/preview.ts:330 does new DOMParser().parseFromString(svgText, 'image/svg+xml') — the XML parser, which surfaces a <parsererror> and, per installSvg, causes the diagram not to be shown at all.

So a fixture can be malformed in a way the suite accepts and the real path rejects. The tests then assert against a DOM the extension would never have built.

It has already happened

While adding template-with-cluster-legacy.svg I wrote a header comment containing --rm. A double hyphen is not legal inside an XML comment, so the document is not well-formed. The full suite stayed green. It was caught only by running an external XML parser over the file by hand:

xml.parsers.expat.ExpatError: not well-formed (invalid token): line 30, column 17

Had that shipped, svg-theme.test.ts would have gone on passing while the webview refused the document outright — the tests asserting confidently about a diagram the extension could not display.

Both fixtures are well-formed today; the gap that let it through is not closed.

Suggested shape

Make the fixtures pass through the same gate the webview uses, so leniency cannot hide a malformed one. Roughly:

• Parse each fixture with DOMParser / image/svg+xml in install() and fail on parsererror, rather than assigning innerHTML; or
• Keep innerHTML for the DOM the assertions need, and add a separate test that every file in test/fixtures/svg/ is well-formed XML.

The first is closer to what the webview does and needs no second list of fixtures to maintain. Worth checking that happy-dom's DOMParser reports parsererror the way a browser does before relying on it — if it does not, the second option is the honest one.

Acceptance

• A malformed fixture fails the suite.
• Reintroducing the exact --rm-in-an-XML-comment defect is a red test, not a green one.