Agents - preserve reply draft when feedback widget rebuilds

[benibenj]

Problem

When typing a reply on a feedback item in the agent feedback editor widget, the reply input could close even though it had text in it. This happened because any unrelated onDidChangeFeedback (or code/PR review state change) tears down all widgets via _clearWidgets and rebuilds them, throwing away the textarea the user was typing in.

Fix

Persist in-progress reply state on the contribution (which survives rebuilds) and have each widget restore it:

• IReplyDraftState is owned by AgentFeedbackEditorWidgetContribution and passed to every AgentFeedbackEditorWidget.
• _startAddingReply writes the textarea value to the shared drafts map on every input event, and clears it on submit / cancel.
• Before disposing widgets, _clearWidgets snapshots whether one of the reply textareas was the active element so the rebuilt widget can refocus it.
• _buildFeedbackItems re-opens the reply input for any comment that has a saved draft, restoring the text and focus.
• Orphaned drafts (whose comment no longer exists) are pruned after each rebuild.
• The teardown blur is ignored (this._disposed) so it doesn't wipe the draft as part of widget disposal.

Behaviour

• Reply input stays open when focus moves elsewhere if it contains text (existing behaviour, now actually upheld across rebuilds).
• Empty reply input still auto-cancels on blur (unchanged).
• Escape and Enter on empty text still cancel (unchanged).
• Caret is placed at the end of restored text so typing continues naturally.

[Copilot]

Pull request overview

This PR fixes a usability issue in the Agents feedback editor widget where an in-progress reply could be lost when the widget is torn down and rebuilt due to unrelated feedback / review state changes. It introduces shared draft state owned by the editor contribution so reply text (and optionally focus) can be restored across rebuilds.

Changes:

• Add a shared reply-draft store (drafts + focusedCommentId) on AgentFeedbackEditorWidgetContribution and pass it into each widget instance.
• Persist reply textarea content into the shared store on input, restore drafts during _buildFeedbackItems, and capture focused reply before teardown so focus can be restored.
• Update the widget fixture to match the new constructor parameter.

Show a summary per file

File	Description
src/vs/sessions/contrib/agentFeedback/browser/agentFeedbackEditorWidgetContribution.ts	Adds shared reply-draft state and restoration logic across widget rebuilds, plus focus snapshotting and orphan draft pruning.
src/vs/sessions/contrib/agentFeedback/test/browser/agentFeedbackEditorWidget.fixture.ts	Updates fixture instantiation to include the new optional draft-state argument.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2