Tooltip instances should be reused when calling `Tooltip.forComponent`
repeatedly for the same component, as introduced in #4736. This did not
work for composite components: the tooltip was stored on the original
component, but the lookup in `Tooltip#getForElement` resolves the
innermost content component instead, so the cached instance was never
found and a new one was created on every call.

```java
Component composite = new MyComposite(); // extends Composite<Span>
Tooltip a = Tooltip.forComponent(composite);
Tooltip b = Tooltip.forComponent(composite);
// a != b before this fix
```

Store the cached tooltip on the innermost component in `forComponent`,
so the write matches the read in `getForElement` (and the existing
`forHasTooltip`, which already resolves the innermost component).

Fixes #9638

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>