Add Symfony LSP diagnostics to CI

[loevgaard]

Integrates symfony lsp:check into the build workflow as a step of the coding-standards job (PHP 8.1, highest deps), next to the YAML and Twig lint steps. It runs symfony lsp:check --format=github, so unknown routes/services/templates/translation keys, deprecated config keys etc. show up as annotations on pull requests.

Repository changes needed for symfony-lsp to find and boot the test application

symfony-lsp discovers an application only if its composer.json requires symfony/framework-bundle, locates the kernel through a PSR-4 entry in that composer.json, and boots it via <project>/vendor/autoload.php. The test application is not a standalone Composer project, hence:

• tests/Application/composer.json now declares the symfony/framework-bundle requirement and the PSR-4 namespace of the kernel. Nothing is ever installed from this file; it is only a marker (as it already was for project-dir resolving).
• tests/Application/vendor is a symlink to the root vendor directory.
• phpunit.xml.dist excludes tests/Application and vendor from test discovery because PHPUnit follows symlinks (without this it would try to load the 500+ test classes shipped in vendor).
• .symfony-lsp.json points symfony-lsp at tests/Application and enables translation diagnostics.
• CLAUDE.md documents the command.

The second commit removes the two deprecated config keys the check flagged in the test application (framework.form.legacy_error_messages, security.enable_authenticator_manager). Both are no-ops on Symfony 6.4.

Verification (local, PHP 8.3, symfony-lsp 0.20.0)

• symfony-lsp check: 0 diagnostics, exit code 0 (2 deprecation warnings before the second commit)
• vendor/bin/phpunit: 3 tests OK, only the plugin's own tests are discovered
• bin/console lint:container (test env) and lint:yaml pass
• ECS, PHPStan and Rector are unaffected by the symlink (they do not follow symlinks)

Running the check locally requires the Symfony CLI 5.20 or newer (symfony lsp:check), or the standalone symfony-lsp binary from https://github.com/symfony/language-tools/releases.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cae313a) to head (311b161).

Additional details and impacted files

@@             Coverage Diff             @@
##              1.14.x       #87   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity         2         2           
===========================================
  Files              2         2           
  Lines             16        16           
===========================================
  Hits              16        16           

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