fix: Use String type for Style.bind return value (#23888) (CP: 25.1)

[vaadin-bot]

This PR cherry-picks changes from the original PR #23888 to branch 25.1.

Original PR description

Change Style.bind(String, Signal) return type from SignalBinding<?> to SignalBinding since the method always accepts a Signal and we always know the value type is String.

This provides better type safety and eliminates the need for casts when using the binding's onChange callbacks.

Changes:

• Style.bind: return SignalBinding instead of SignalBinding<?>
• BasicElementStyle.bind: update implementation
• ImmutableEmptyStyle.bind: update implementation
• AbstractPropertyMap.bindSignal: make generic with type parameter
• StyleBindTest: update tests to use typed BindingContext
• StyleBindTest: add test to verify typed return value

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
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

[github-actions]

Test Results

 1 384 files  ±0   1 384 suites  ±0   1h 26m 44s ⏱️ + 1m 30s
 9 909 tests +1   9 838 ✅ +1  71 💤 ±0  0 ❌ ±0 
10 382 runs  +1  10 302 ✅ +1  80 💤 ±0  0 ❌ ±0 

Results for commit 871103b. ± Comparison against base commit 223054a.

[vaadin-bot]

This ticket/PR has been released with Vaadin 25.1.0-rc2.