feat(js-bridge): add bridgeEvents API and typed handler helpers

[parkwoocheol]

Connection with issue(s)

Resolve issue #2793

Connected to #???

Testing and Review Notes

1. Static checks

• flutter analyze

2. Bridge events API verification (Advanced > Controllers)

• Validate JS -> Dart event delivery (bridgeEvents.emit in JS, Dart listener receives payload)
• Validate Dart -> JS event delivery (Dart emit, JS listener receives payload)
• Validate on/off/hasListener behavior

3. Typed handler helper verification

• Validate addSerializedJavaScriptHandler<TRequest, TResponse>(...) roundtrip
• Validate addJsonJavaScriptHandler<TRequest, TResponse>(...) roundtrip
• Confirm existing addJavaScriptHandler behavior is unchanged

4. Platform validation

• Android: runtime/integration validation completed
• Web: build/runtime path validation completed
• iOS/macOS: additional runtime validation required in local Xcode/CocoaPods environment

5. Regression checkpoints

• No hardcoded bridge global name (runtime bridge name is used)
• No bypass of existing JS communication/security policies
• New APIs are additive only (no breaking changes)

Screenshots or Videos

To Do

• double check the original issue to confirm it is fully satisfied
• add testing notes and screenshots in PR description to help guide reviewers
• request the "UX" team perform a design review (if/when applicable)