FORNO-451: Add Redo for inline text edits

[kat3samsin]

Part of FORNO-451

Partner PR: BigSky 6233

Proposed Changes

• Show Updated with inline Undo only for successful text-content edits.
• After Undo, show Reverted with Redo and swap the same provider-owned checkpoint for Big Sky and Jetpack.
• Hide the action for structural edits, CSS changes, and no-op outcomes.
• Preserve the action when Agenttic replaces the streamed tool result with final prose.
• Hide Undo and Redo after Gutenberg's native Undo (ctrl+Z) changes the editor history.
• Hide stale Undo and Redo controls after a new user message or when conversation history is loaded.
• Remove the background from Updated while keeping the green text.

Why are these changes being made?

• This completes the inline text-edit action introduced in Jetpack AI: show block edit outcomes with Undo #113317 and keeps its controls limited to the current editor and chat state.

Testing Instructions

1. On a sandbox with big-sky-plugin#6233, ask the agent to change paragraph text.
2. Confirm Updated appears as green text with no background and Undo is available.
3. Click Undo. Confirm the original text returns and the action changes to Reverted with Redo.
4. Click Redo. Confirm the edited text returns and the action changes to Updated with Undo.
5. Ask the agent to move, insert, or delete a block, change CSS, or make no change. Confirm NO Updated or Undo action appears.
6. Confirm the action remains visible when the final assistant response replaces the streamed tool result.
7. After an AI text edit, press Ctrl+Z or Cmd+Z. Confirm inline Undo and Redo disappear and stay hidden after native Redo.
8. Send another user message and confirm controls on the prior turn disappear. Start a new chat, reopen the old conversation from History, and confirm its controls stay hidden.

Automated checks:

• Focused Agents Manager suites for native Undo and history invalidation pass: 54 tests.
• Four focused Agents Manager and Jetpack AI Sidebar suites pass: 288 tests.
• The full Jetpack AI Sidebar suite passes: 17 suites, 379 tests.
• A full Agents Manager run also passed once; a rerun hit the existing order-dependent tracks.test.ts failure, and that file passes alone: 18 tests.
• Scoped ESLint, Stylelint, Prettier, and git diff --check pass.
• @automattic/jetpack-ai-sidebar typecheck and build pass.
• The Agents Manager production widget build passes with local source aliases.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• For UI changes, have you tested the affected components in dark mode?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have you tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

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[a8ci18n]

This Pull Request is now available for translation here: https://translate.wordpress.com/deliverables/34863821

Some locales (Hebrew, Japanese) have been temporarily machine-translated due to translator availability. All other translations are usually ready within a few days. Untranslated and machine-translated strings will be sent for translation next Monday and are expected to be completed by the following Friday.

Thank you @kat3samsin for including a screenshot in the description! This is really helpful for our translators.