[build-tools] Proxy React Native Maven artifacts

[sjchmiela]

Why

React Native 0.81 and newer support ENTERPRISE_REPOSITORY for stable Hermes, React Native Core, and React Native Dependencies artifacts. React Native 0.87 also reads this setting in its experimental Swift Package Manager artifact downloader.

Set a system fallback so React Native downloads during pod install use the existing EAS CocoaPods cache without replacing a repository configured by the user.

React Native source: https://github.com/facebook/react-native/blob/v0.87.0/packages/react-native/scripts/cocoapods/rncore.rb

How

During pod install, set ENTERPRISE_REPOSITORY to <EAS_BUILD_COCOAPODS_CACHE_URL>/repo1.maven.org/maven2 only when the user did not set it.

The existing proxy mechanisms remain active:

• EAS_BUILD_COCOAPODS_CACHE_URL stays set, so the CocoaPods plug-in handles all other supported hosts.
• An already proxied Maven URL is not proxied a second time.
• If that cache download fails, the CocoaPods plug-in reconstructs the Maven Central URL and retries it directly. This preserves the existing origin fallback.
• The curl wrapper leaves the already proxied URL unchanged because it only rewrites URLs that start with https://repo1.maven.org/maven2.

This change does not remove the CocoaPods plug-in or curl wrapper.

Test Plan

• Added build-tools tests for the system fallback, user precedence, and disabled-cache behavior.
• Added expo-cocoapods-proxy tests for an already proxied Maven URL and its direct-origin fallback.
• yarn workspace @expo/build-tools test pod.test.ts --runInBand
• bundle exec rspec spec/expo_cocoapods_proxy/proxy_spec.rb
• yarn build
• yarn fmt:check
• yarn lint (0 errors; existing unrelated warnings remain)

[github-actions]

⏩ The changelog entry check has been skipped since the "no changelog" label is present.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.39%. Comparing base (2a7cb5e) to head (b544528).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4196      +/-   ##
==========================================
+ Coverage   63.39%   63.39%   +0.01%     
==========================================
  Files        1027     1027              
  Lines       46881    46888       +7     
  Branches     9834     9835       +1     
==========================================
+ Hits        29715    29722       +7     
  Misses      17064    17064              
  Partials      102      102              

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