React Native: fix linker error when plugin-react-native-span-access is not installed

[hamudi-ahmed]

Goal

The BugsnagReactNativeAppStartPlugin implementation is provided by the optional plugin-react-native-span-access module, but we still had a direct reference to the plugin in the main React Native SDK, causing linker errors when the optional module is not installed (as reported in #785)

This PR fixes the issue by removing the plugin reference and implementing a BugsnagPerformanceAppStartProvider and BugsnagPerformanceAppStartRegistry similar to the Android implementation.

Design

The BugsnagReactNativeAppStartPlugin now registers itself with the BugsnagPerformanceAppStartRegistry during the install phase, and the main SDK now looks for a registered BugsnagPerformanceAppStartProvider instead of calling the plugin instance directly.

Testing

Existing app start tests covered by CI, plus some additional manual testing to ensure that projects still build and run successfully when the optional module is not installed.

[github-actions]

Browser bundle size

NPM build

	Package
Before	235.34 kB
After	235.34 kB
±	No change

CDN build

	Unminified	Minfied	Minified + gzipped
Before	119.42 kB	46.16 kB	13.29 kB
After	119.42 kB	46.16 kB	13.29 kB
±	No change	No change	No change

Code coverage

Coverage values did not change👌.

Total:

Lines	Branches	Functions	Statements
85.05%(+0%)	63.82%(+0%)	75.66%(+0%)	81.83%(+0%)

Generated against 8802f00 on 17 February 2026 at 10:51:18 UTC