feat: 5308 add Rich Text field type for schema forms

[ihar-tsykala]

Description:

Every text field in a Guardian schema was plain text. An author writing instructions into a form had no way to mark a heading, emphasise a warning, or link to an external page: everything arrived as one flat paragraph, which readers misread.

Add a Rich Text field type. In the schema it is an ordinary string field carrying customType: 'richText', so the value stays a plain JSON string and nothing downstream needs a new data type. It is stored as Markdown, which travels safely to Hedera and stays readable on its own. A policy form shows a small editor instead of a textarea — bold, italic, both list kinds, H1–H3 and links — and the document viewer, the documents grid and the Indexer render that Markdown as formatted output, with links opening in a new tab. Pasted content is reduced to the same set, so a copy from a word processor cannot bring fonts, tables or images into a document. The type is registered in the field-type picker and the Excel dictionary, and preset values use the same editor.

Underline is deliberately absent: Markdown cannot store it, and raw HTML is not mixed into the value. Nothing changes for a field without that custom type, and stored values are never rewritten.

Fixes #5308

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[github-actions]

Test Results

 33 files  ±0   66 suites  ±0   4m 33s ⏱️ +23s
 35 tests ±0   35 ✅ ±0  0 💤 ±0  0 ❌ ±0 
173 runs  ±0  173 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 161602c. ± Comparison against base commit b398d6c.

♻️ This comment has been updated with latest results.

[Pyatakov]

Please, address the comments below (more details added inline).

• frontend/src/app/modules/schema-engine/rich-text-editor/markdown.ts:7 – escapeHtml leaves " unescaped, so a markdown link URL can inject attributes into the generated <a>
• indexer-frontend/src/app/components/schema-form-view/markdown-view.ts:7 – Same unescaped-quote attribute injection in the indexer copy of escapeHtml
• frontend/src/app/modules/schema-engine/rich-text-editor/rich-text-editor.component.ts:474 – Plain-text paste collapses every line break into a single line
• frontend/src/app/modules/policy-engine/policy-viewer/blocks/documents-source-block/documents-source-block.component.html:77 – getRichTextCellText parses a fresh HTMLDocument per rich-text cell on every change detection cycle
• interfaces/src/helpers/schema-json.ts:187 – Redundant richText special case – the loop below already resolves it