Normalize markdown and richtext editor field handling

[bobbwal]

Description

Normalizes editor field handling so markdown becomes the canonical markdown field type, while mdxeditor and easymde remain supported aliases. This also fixes editor provider scoping, schema-backed field resolution, and a few related admin form regressions uncovered during review.

Fixes #569
Fixes #674
Supersedes PR #570

Changes

• Make markdown the canonical markdown field type in types, admin collection editing, and docs
• Keep mdxeditor and easymde working as compatibility aliases
• Scope TinyMCE and EasyMDE initialization to the correct editor provider to prevent cross-binding
• Preserve canonical richtext as the TinyMCE-backed default rich text type
• Normalize tinymce back to richtext in the admin collection editor flow so schemas round-trip safely
• Fix schema-backed content field resolution so:
  • format: 'richtext' renders via the richtext path instead of a plain text input
  • enum-backed string fields resolve to select and keep their options
• Restore explicit fallback warnings/textarea behavior when TinyMCE or the markdown editor plugin is inactive
• Add/update docs for the normalized field type model
• Add focused unit and Playwright regression coverage for markdown/richtext/select behavior

Testing

• npm run type-check
• npm test --workspace=packages/core -- dynamic-field-extended.test.ts admin-content-field-types.test.ts admin-collections-field-types.test.ts
• BASE_URL=http://localhost:8787 npx playwright test tests/e2e/60-markdown-field-type.spec.ts --config tests/ playwright.config.ts
• BASE_URL=http://localhost:8787 npx playwright test tests/e2e/61-richtext-field-type.spec.ts --config tests/ playwright.config.ts

Unit Tests

• Added/updated unit tests
• All unit tests passing

E2E Tests

• Added/updated E2E tests
• Targeted E2E tests passing

Screenshots/Videos

• None

Checklist

• Code follows project conventions
• Tests added/updated and passing
• Type checking passes
• No console errors or warnings
• Documentation updated (if needed)