Add "Copy for LLM" button for error occurrences

[rickychilcott]

A rework of upstream fractaledmind/solid_errors#97 by @bvalentino. Same idea — one click gets you an error in a form you can paste into an LLM — different plumbing.

What changed vs. fractaledmind#97

A real URL instead of an inline <template>. GET /errors/:error_id/occurrences/:id.md renders the prompt. fractaledmind#97 embedded the whole document in every error page, which meant a second Backtrace.parse plus a File.open per application frame on every page view whether or not anyone clicked. Now that work only happens when someone asks for it, and you can curl the URL or paste it to a coworker.

A view, not a model method. fractaledmind#97 added Error#to_markdown, 35 lines of md << "...". This uses app/views/solid_errors/occurrences/show.text.erb, so the formatting sits next to the HTML formatting it mirrors and the model is untouched.

XML-ish tags and instructions, matching the shape Honeybadger emits. Delimited blocks also dodge a real escaping problem: an exception message containing ## or a source snippet containing triple-backticks corrupts a heading-based markdown document, and solid_errors stores raw exception messages.

Per-occurrence, not "the latest one". A button in each occurrence's summary copies that failure; the action-bar button copies the newest.

Degrades instead of dying. fractaledmind#97's .catch never fires — on a non-secure origin navigator.clipboard is undefined, so navigator.clipboard.writeText throws a TypeError synchronously and the button silently does nothing. Plenty of people reach a mounted engine dashboard over plain HTTP. Here the buttons are anchors and the handler only calls preventDefault() when it can actually reach the clipboard, so a non-secure origin, a JS failure, or a cmd-click all fall through to the readable page.

Notes

• Served as text/plain, not text/markdown — browsers have no renderer for text/markdown and download the file instead of showing it, which would break both the navigable URL and the fallback. GitHub serves raw .md the same way.
• Rendering through the text format also means ActionView's ERB handler skips HTML escaping, so backtraces and messages survive verbatim.
• The engine is now mounted in the dummy app, which is what makes request tests possible at all — there were none before.

Sample output

Fix this error from the Dummy application, reported by Solid Errors:

<fault id="2" environment="test">
<class>ArgumentError</class>
<message>kaboom</message>
<severity>warning</severity>
<source>application</source>
<occurred-at>2026-07-31T13:13:38Z</occurred-at>
<occurrence-count>1</occurrence-count>
<project-root>/path/to/app</project-root>
<backtrace>
[PROJECT_ROOT]/app/models/boom.rb:3:in `Boom.explode'
</backtrace>
<source-context>
<file path="[PROJECT_ROOT]/app/models/boom.rb" line="3">
1: class Boom
2:   def self.explode
3:     raise ArgumentError, "kaboom"
4:   end
</file>
</source-context>
<context>
{
  "controller": "UsersController",
  "action": "show"
}
</context>
</fault>

Investigate and fix the root cause of this error:

1. Use the backtrace and source context to locate where the error was raised in this codebase.
...

Testing

test/integration/solid_errors/markdown_test.rb covers the route, content type, unescaped output, the 404 on a mismatched error/occurrence pair, and the links on the error page. Full suite: 8 runs, 34 assertions, 0 failures.

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