Add 'Show Outdated' filter to Comments panel

[Copilot]

Description

Adds a third filter option to the Comments panel filter menu alongside "Show Resolved" and "Show Unresolved". When unchecked, hides comments with CommentThreadApplicability.Outdated.

Changes

• Filter infrastructure (commentsViewActions.ts): Added CONTEXT_KEY_SHOW_OUTDATED, extended CommentsFilters with showOutdated property, registered toggleOutdatedComments action
• Filter logic (commentsTreeViewer.ts): Extended filterCommentNode() to check threadRelevance against CommentThreadApplicability.Outdated
• State persistence (commentsView.ts): Added showOutdated to CommentsViewState for session persistence
• Tests (commentsView.test.ts): Added test case verifying filter behavior with Current and Outdated threads

Filter defaults to enabled (shows all comments). Comments without applicability info are always shown.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Filter out 'outdated' comments</issue_title>
<issue_description>

Currently in the comments panel there is an option to filter comments by "Show Resolved" and "Show Unresolved". It would be great if there was an option to filter outdated comments ("Show Outdated") as well.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes microsoft/vscode-pull-request-github#8287

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