Update plugins/builtin example to use php83 to avoid flaky tests

[savil]

Problem

The cli-tests on main failed on Oct 24 due to a flaky test in the php-dom extension for PHP 8.2.29:

building '/nix/store/4m46w082habdfswbjagviz3b43g86c8z-php-dom-8.2.29.drv'...
error: builder for '/nix/store/4m46w082habdfswbjagviz3b43g86c8z-php-dom-8.2.29.drv' failed with exit code 2;

FAILED TEST SUMMARY
GH-10234 (Setting DOMAttr::textContent results in an empty attribute value.) [tests/gh10234.phpt]

Analysis

Evidence of PHP version difference between runs:

Oct 23 run - ✅ SUCCESS

• Run: 18735766466
• Job: test (is-main, ubuntu-latest, project-tests-only, 2.12.0)
• PHP Version: 8.3.3
• Log excerpt:

  copying path '/nix/store/2c7asm2fwnlziayhmyhywldsym5mwpg9-php-dom-8.3.3' from 'https://cache.nixos.org'...
  

Oct 24 run - ❌ FAILURE

• Run: 18774433913
• Job: test (is-main, ubuntu-latest, project-tests-only, 2.12.0)
• PHP Version: 8.2.29
• Log excerpt:

  building '/nix/store/4m46w082habdfswbjagviz3b43g86c8z-php-dom-8.2.29.drv'...
  error: builder for '/nix/store/4m46w082habdfswbjagviz3b43g86c8z-php-dom-8.2.29.drv' failed with exit code 2;
  
  FAILED TEST SUMMARY
  GH-10234 (Setting DOMAttr::textContent results in an empty attribute value.) [tests/gh10234.phpt]
  

The issue is specific to PHP 8.2.29 in nixpkgs-unstable.

Solution

Update the examples/plugins/builtin example from php82 to php83.

This is a conservative approach because:

• ✅ PHP 8.3.3 already proven to work (Oct 23 success)
• ✅ Maintains upstream testing (doesn't skip tests)
• ✅ Minimal change (one line)
• ✅ Avoids the problematic PHP 8.2.29 version

Changes

• examples/plugins/builtin/devbox.json: Change from plugin:php82 to plugin:php83

Alternative Considered

Initially considered adding doCheck = false to the PHP plugin flake, but updating the PHP version is cleaner and more conservative since PHP 8.3 is already known to work.

Testing

This PR will trigger cli-tests. The examples/plugins/builtin test should pass with PHP 8.3.