fix: reuse tooltip instance for composite components

[web-padawan]

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.

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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🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vaadin-bot]

Hi @web-padawan and @web-padawan, when i performed cherry-pick to this commit to 25.1, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick 8fda378
error: could not apply 8fda378... fix: reuse tooltip instance for composite components (#9639)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".