fix(chat): elaborate the bidirectionality of the messages array - #438

#535, #491 (#556)

* docs(jp): sync Angular JP docs with EN PRs #522, #438, #535, #491

Manual replay of the four failed "Sync Japanese Documentation (Angular)"
runs (33877005832, 34238718782, 34250163709, 34329511530). Each died in
the Copilot CLI step with transient_bad_request before doing any work;
the workflows have since been recompiled with gh-aw v0.88.7 in #555.

- chat: translate the `messages` row and the new bidirectional messages
  collection section (#438)
- theming-mcp: add the WCAG contrast note linking to the new
  Accessibility topic (#522)
- palettes: hsla() contrast variables in the CSS excerpt (#522)
- toc: add Themes > Accessibility; Button now points at
  inputs/button.mdx (#522, #491)
- retarget IgxButton links in dialog, time-picker, tooltip, roundness,
  material/bootstrap theming and the row-editing/search grid templates
  to the xplat-generated inputs/button page (#491, #535)
- remove the hand-authored button.mdx and layouts/avatar.mdx, superseded
  by the xplat-generated pages exactly as EN did (#491, #522)
- xplat jp toc: lift the Angular exclude on Button, Icon, Icon Button
  and Avatar and add Themes > Accessibility, mirroring the EN xplat toc
  so the Angular JP site gains the generated pages the Angular toc now
  references
- add the jp xplat themes/accessibility.mdx translation (target of the
  new toc entry and of the theming-mcp note)
- mirror the new button anatomy / do-don't images into jp/images, which
  the Angular build resolves per language via @xplat-images

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

* docs(jp): sync xplat JP docs with EN PRs #522, #438, #535, #491

Manual replay of the four failed "Sync Japanese Documentation (xplat)"
runs (33877005838, 34238718784, 34250163990, 34329511576), which died in
the Copilot CLI step exactly like their Angular counterparts. Change set
is the EN diff under docs/xplat/src/content/en/ since the last successful
xplat sync (d346871), filtered to TOC-covered files per the workflow.

- inputs/button: re-mirror the rewritten EN topic (anatomy, getting
  started, usage/variants/states/layout/size/download/do-don't,
  properties, styling incl. Tailwind, accessibility, troubleshooting,
  dependencies, related, FAQ), reusing existing JP prose where EN kept it.
  The Getting Started link is split per platform like the JP badge and
  button-group pages, because the JP xplat toc still excludes
  general-getting-started.mdx for Angular (EN does not)
- inputs/button-group: Angular-specific PlatformBlocks throughout,
  {PackageCommon} install block, per-platform properties tables, API
  references and dependencies, new Related/FAQ content
- inputs/badge: {PackageCommon} install block, Angular import sentence,
  CSS variables table, troubleshooting/FAQ updates
- interactivity/accessibility-compliance: replace the Section 508 / WCAG
  matrices with the new standards, criteria, per-component documentation
  index (JP heading anchors), configuration, VPAT and FAQ sections
- interactivity/chat: Blazor MessageCreated guidance, per-platform
  property names, bidirectional messages collection section
- themes/overview: configureTheme variant argument, new Customizing with
  Sass section; themes/accessibility now anchors to it
- themes/palettes: runtime relative-color contrast, seed variables,
  --ig-wcag-* excerpt, contrast-level note
- ai/theming-mcp: WCAG contrast note

Not mirrored: the EN toc href fix for grids/grid/theming-grid.mdx (#535)
only corrected EN after a July file move (#355) that was never applied to
JP; JP still keeps the page at grids/theming-grid.mdx consistently.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

* review: apply native check fixes (fix wording inconsistencies)

* review: apply native check fixes (reword literal "boundaries" translation in troubleshooting intros and known-limitations lead-in)

* review: apply native check fixes (reword literal "boundaries" translation in the Before You Start lead-in)

* review: apply native check fixes (restore accessibility anchors on carousel and tooltip links)

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: jsakamotoIGJP <jsakamoto@infragistics.com>