Enable branch coverage in SimpleCov configuration

- Add enable_coverage :branch to SimpleCov.start block
- Improves test coverage reporting by tracking branch coverage
- Helps identify untested code paths and conditional logic

Author: ericproulx

.simplecov

@@ -11,5 +11,6 @@ if ENV['GITHUB_USER'] # only when running CI
 end
 
 SimpleCov.start do
+  enable_coverage :branch
   add_filter '/spec/'
 end

CHANGELOG.md

@@ -17,6 +17,7 @@
 * [#2601](https://github.com/ruby-grape/grape/pull/2601): Refactor route_setting internal usage to use inheritable_setting.route for improved consistency and performance - [@ericproulx](https://github.com/ericproulx).
 * [#2602](https://github.com/ruby-grape/grape/pull/2602): Remove `namespace_reverse_stackable` from public DSL interface and use direct inheritable_setting access - [@ericproulx](https://github.com/ericproulx).
 * [#2603](https://github.com/ruby-grape/grape/pull/2603): Remove `namespace_stackable_with_hash` from public interface and move to internal InheritableSetting - [@ericproulx](https://github.com/ericproulx).
+* [#2604](https://github.com/ruby-grape/grape/pull/2604): Enable branch coverage  - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 #### Fixes