feat: MUI 4 to BUI migration - replace MUI Tabs with BUI Tabs

[schultzp2020]

Summary

Add a codemod that migrates the common MUI tabs stack to BUI Tabs, TabList, Tab, and TabPanel.

The codemod should handle both the simple core Tabs + Tab pattern and the Material UI Lab TabContext / TabPanel pattern when the selected key model is explicit in the source.

Detection Criteria

• Tabs and Tab imports from @material-ui/core
• TabContext, TabList, or TabPanel imports from @material-ui/lab
• static value props on tabs and panels, plus onChange handlers that select a tab key

Transformation Logic

1. Replace the MUI tab imports with Tabs, TabList, Tab, and TabPanel from @backstage/ui.
2. Map MUI value identifiers to BUI id props on Tab and TabPanel.
3. Rewrite the root selection API to selectedKey / defaultSelectedKey and onSelectionChange when the mapping is mechanical.
4. Move label text into the Tab children.
5. Preserve panel children and ordering.
6. Insert TODO(backstage-codemod): verify custom tab orientation or selection logic manually for vertical tabs, custom a11y wrappers, or complex index-based state.

Before / After Example

TabContext-based tabs:

// Before
<TabContext value={tab}>
  <TabList onChange={handleChange}>
    <Tab label="Overview" value="overview" />
  </TabList>
  <TabPanel value="overview">Content</TabPanel>
</TabContext>

// After
<Tabs defaultSelectedKey="overview">
  <TabList>
    <Tab id="overview">Overview</Tab>
  </TabList>
  <TabPanel id="overview">Content</TabPanel>
</Tabs>

Notes / Edge Cases

• The codemod should not try to preserve index-based helper utilities if the source already exposes stable string values.
• Vertical and custom-scrollable tab layouts are valid but may need manual follow-up after the structural conversion.
• Representative Backstage evidence comes from InspectEntityDialog and the migration skill examples.
• This issue is separate from the dialog-shell codemod even though they often appear in the same files.

Changeset (when implementing)

• Package: @backstage/migrate-mui-tabs-to-bui-tabs
• Bump: minor (initial release)
• Summary example: Add codemod to migrate mui tabs to bui tabs for the Backstage MUI 4 to BUI migration

Implementation notes

• Target directory: codemods/misc/migrate-mui-tabs-to-bui-tabs/
• Branch/worktree: .worktrees/feat/mui4-to-bui/migrate-mui-tabs-to-bui-tabs
• Open PR when ready; one PR per codemod