test: enable RSpec --profile to surface slowest package tests (#3176)

## Summary
- Uncomments `config.profile_examples = 10` in
`react_on_rails/spec/react_on_rails/spec_helper.rb` so each run prints
the 10 slowest examples.
- Enables data-driven optimization of the ~24 minute package test suite
— prerequisite for targeted perf work on generator tests, redundant
`before(:all)` blocks, etc.

Fixes #3132

## Test plan
- [x] `bundle exec rubocop
react_on_rails/spec/react_on_rails/spec_helper.rb` — no offenses
- [x] Ran a sample spec file (`utils_spec.rb`) locally; confirmed "Top
10 slowest examples" block appears at the end of the run
- [ ] CI green

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: test-only configuration change that only affects RSpec
output and adds minor runtime overhead from collecting timings.
> 
> **Overview**
> Enables RSpec profiling in
`react_on_rails/spec/react_on_rails/spec_helper.rb` by turning on
`config.profile_examples = 10`, so each spec run reports the 10 slowest
examples/example groups to help target test-suite performance work.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
82417ab. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Tests**
* Enhanced test performance profiling to identify and report the slowest
test cases during spec runs.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: justin808

react_on_rails/spec/react_on_rails/spec_helper.rb

@@ -121,10 +121,10 @@
   #     config.default_formatter = 'doc'
   #   end
   #
-  #   # Print the 10 slowest examples and example groups at the
-  #   # end of the spec run, to help surface which specs are running
-  #   # particularly slow.
-  #   config.profile_examples = 10
+  # Print the 10 slowest examples and example groups at the
+  # end of the spec run, to help surface which specs are running
+  # particularly slow.
+  config.profile_examples = 10
   #
   #   # Run specs in random order to surface order dependencies. If you find an
   #   # order dependency and want to debug it, you can fix the order by providing