`mermaid.render()` returns SVG aimed at inline HTML, and its sanitizer
gives back a `<br/>` written in a label as an open `<br>`. That string is
not well-formed XML, so the strict `image/svg+xml` parse in
`normalizeSvgSize` yields the parsererror document instead of an `<svg>`,
takes the `el.tagName !== 'svg'` early return, and hands the *unnormalized*
string back. The diagram `<img>` then gets it as a `data:` URI, which Blink
decodes as strict XML and rejects for the same reason — so the message shows
a broken image whose alt text reads "Mermaid diagram", with nothing thrown
and nothing written to any log.
Close HTML void tags before the markup reaches either XML parse. The pass is
generic rather than a `<br>` special case, because the same class of input
reaches `<hr>`, `<img>`, `<input>`, …, and it is idempotent: already-closed
tags and non-void tags (including names that merely start with a void tag,
such as `<break>`) pass through untouched.
Fixes#133089
Verified by replaying 32 real ```mermaid fences from a session database
through the app's own renderer chunks in Electron 40.10.2 / Chromium 144:
24/32 fail before this change — every failure has `<br/>` in a label, with a
perfect 1:1 correlation — and 31/31 renderable diagrams succeed after it. The
32nd is an invalid Mermaid source that correctly keeps falling back to its
source block.
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