Fix links of footnotes referenced more than once

What does this MR do and why?

When a footnote is referenced more than once, the engine gives the later references ids such as fnref-1-2, all linking to #fn-1. FootnoteFilter, which adds a random suffix to footnote ids, took the footnote name from the reference id, found no footnote named 1-2 and skipped the reference, so every reference after the first linked to a missing anchor and only the first back-reference got the suffix.

This MR takes the footnote from the href and adds the suffix to every back-reference. It finds the footnote by comparing ids instead of building a CSS selector from its name, so a name with a space or a line break in hand-written HTML no longer raises Nokogiri::CSS::SyntaxError.

This MR needed !265760 (merged), which merged on 2026-10-30. The rich text editor read a reference's name from its id, and its pattern would read the fixed references as [^null]; !265760 (merged) makes it read the href. GitLab.com runs mixed versions during a deploy, so the editor change went first.

• Cached HTML is fixed when next rendered; I did not bump CACHE_COMMONMARK_VERSION, which has its own rollout procedure.
• No feature flag: a footnote referenced once renders as before.

The new specs fail on master (4 RSpec examples: 3 in the gitlab-banzai gem, where master moved FootnoteFilter, and 1 in the monolith) and pass with this MR.

Specs and checks I ran

• gems/gitlab-banzai, with the gem's own bundle: 437 examples, 0 failures; with master's filter the 3 new examples in spec/banzai/filter/footnote_filter_spec.rb fail.
• spec/lib/banzai/filter/footnote_filter_spec.rb (6 examples; with master's filter the new one fails), the sanitization, emoji and Markdown filter specs, the full and GFM pipeline specs, spec/initializers/banzai_spec.rb and spec/features/markdown/markdown_spec.rb: 271 examples, 0 failures.
• RuboCop on the three files and scripts/lint/commit_linter.rb: clean. Ruby 3.3.11 throughout.
• The engine renders repeated footnotes the same with gitlab-glfm-markdown 0.0.45 and 0.0.47, and the full pipeline example passes with 0.0.48, the version in master's Gemfile.lock now.

References

Closes #636343 (closed)

Depended on !265760 (merged), merged on 2026-10-30.

Screenshots or screen recordings

No visual change. For a[^1] b[^1] and [^1]: note, with GITLAB_TEST_FOOTNOTE_ID=42:

Element	Before	After
Second reference	href="#fn-1", no footnote-ref	href="#fn-1-42" id="fnref-1-2-42", footnote-ref
Its back-reference	href="#fnref-1-2", no footnote-backref	href="#fnref-1-2-42", footnote-backref
Rich text editor output, with !265760 (merged)	b[<sup>1</sup>](#fn-1)	b[^1]

How to set up and validate locally

1. Create an issue with this description:

   First reference[^1] and second reference[^1].
   
   [^1]: The note.

2. Select the second superscript 1: on master nothing happens; with this MR it scrolls to the note.

3. Edit the description in the rich text editor (type a character and delete it), then switch back to plain text: on master the second reference becomes [<sup>1</sup>](#fn-1); with this MR it stays [^1].

I checked step 3 with the editor's serializer in Jest, not in a browser.

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