fix: remove hardcoded default testIDs from components

[k0ndee]

Motivation

Several components fell back to a hardcoded default testID (e.g. testID = 'button', testID = 'card', testID = 'search-bar') whenever a consumer didn't pass one explicitly. This meant every instance of a component rendered without an explicit testID still emitted a fixed, non-configurable testID into the tree leading to duplicate/colliding testIDs when multiple instances of the same component render on screen.

This PR removes all hardcoded default testID values so that testID is undefined unless a consumer explicitly provides one. Suffixed child testIDs (e.g. `${testID}-container`) are now only rendered when a testID is explicitly given, instead of always deriving from the removed default.

Along the way this surfaced two latent bugs that the hardcoded defaults had been masking:

• IconButton never forwarded its testID down into the inner icon, relying on CrossFadeIcon's own default instead. Now it forwards ${testID}-icon explicitly.
• FAB.Menu's trigger wrapper put testID on a non-interactive positioning View instead of the actual pressable Shell, which only worked before because Shell had its own hardcoded default in the right place.

Affected components: Button, Card, Chip, Surface, Modal, Menu, Menu.Item, IconButton, Searchbar, CrossFadeIcon, ProgressBar, Appbar.Header, Appbar.Content, Drawer.CollapsedItem, FAB, FAB.Extended, FAB.Menu, BottomNavigation, BottomNavigationBar.

Test plan

• yarn typecheck passes.
• yarn lint passes.
• yarn jest passes (55 suites, 732 tests, 169 snapshots — snapshots updated to reflect the removed default testID attributes).
• Updated unit tests that relied on the old default testID strings (Button, Card, Menu, Menu.Item, Drawer.CollapsedItem, FAB.Menu, ListItem, Appbar, BottomNavigation) to pass an explicit testID where the assertion actually needs one.
• No behavior change for consumers who already pass an explicit testID, only the implicit-default fallback is removed.

[JKobrynski]

@k0ndee we've got a bunch of conflicts

[JKobrynski]

The removed defaults need a bullet in the migration guide's "Test IDs" section (docs/6.x/docs/guides/migration.md) —- it already lists the ones Surface and Appbar.Header lost. Otherwise anyone selecting button, card or search-bar in their e2e tests loses those matches silently.

[k0ndee]

Good point. Added information to migration guide.

[JKobrynski]

Five other components build child testIDs without this guard, so they render a literal
undefined-text - visible in the committed snapshots today. Checkbox/CheckboxItem.tsx:177, DataTable/DataTableCell.tsx:111, List/ListItem.tsx:245, SegmentedButtons/SegmentedButtonItem.tsx:227 and Snackbar.tsx:341. Worth including them?

[k0ndee]

Missed them, included in the new commit.