ref(nav): extract header actions, use pageframe slots (#113083)

Refactors the Issue Details header actions:
- extracts actions into a self-contained component
- switch control flow to `if-return` over inline ternaries
- introduces a `MaybeTopBarSlot` component that does slot projection
behind the `page-frame` flag

| Before | After |
|--------|--------|
| <img width="1280" height="298" alt="issue-detail-before"
src="https://github.com/user-attachments/assets/9ff4ffc5-dec1-417f-8aa9-3d7f8291c9aa"
/> | <img width="1280" height="346" alt="issue-detail-after"
src="https://github.com/user-attachments/assets/1b38f231-810a-4cb3-9da5-cd3af5b1c188"
/> |

Closes DE-1134

Author: natemoo-re

static/app/views/issueDetails/actions/newIssueExperienceButton.spec.tsx

@@ -1,4 +1,4 @@
-import {createContext, useContext} from 'react';
+import {createContext} from 'react';
 
 import type {TourContextType} from 'sentry/components/tours/tourContext';
 
@@ -30,11 +30,3 @@ export const ISSUE_DETAILS_TOUR_GUIDE_KEY = 'tour.issue_details';
 
 export const IssueDetailsTourContext =
   createContext<TourContextType<IssueDetailsTour> | null>(null);
-
-export function useIssueDetailsTour(): TourContextType<IssueDetailsTour> {
-  const tourContext = useContext(IssueDetailsTourContext);
-  if (!tourContext) {
-    throw new Error('Must be used within a TourContextProvider<IssueDetailsTour>');
-  }
-  return tourContext;
-}

static/app/views/issueDetails/actions/newIssueExperienceButton.tsx

@@ -101,30 +101,10 @@ describe('StreamlinedGroupHeader', () => {
         screen.getByRole('button', {name: 'Modify issue assignee'})
       ).toBeInTheDocument();
       expect(screen.getByText('Leander')).toBeInTheDocument();
-      expect(
-        screen.getByRole('button', {name: 'Manage issue experience'})
-      ).toBeInTheDocument();
       expect(screen.getByRole('button', {name: 'Resolve'})).toBeInTheDocument();
       expect(screen.getByRole('button', {name: 'Archive'})).toBeInTheDocument();
     });
 
-    it('displays new experience button if flag is set', async () => {
-      render(
-        <StreamlinedGroupHeader
-          {...defaultProps}
-          group={group}
-          project={project}
-          event={null}
-        />,
-        {
-          organization,
-        }
-      );
-      expect(
-        await screen.findByRole('button', {name: 'Manage issue experience'})
-      ).toBeInTheDocument();
-    });
-
     it('displays share icon if issue has been shared', async () => {
       render(
         <StreamlinedGroupHeader