refactor(autoload): add initial PSR-4 support

[JohnVillalovos]

Establish a Composer PSR-4 autoload mapping for the LibreBooking namespace rooted at the new src/ directory and migrate the first component: the Microsoft OAuth callback handler.

• Map LibreBooking\ to src/ in composer.json, leaving the legacy lib/ tree on the existing include system so the gradual migration passes composer dump-autoload --optimize --strict-psr.
• Split MicrosoftOAuthCallback, MicrosoftOAuthCallbackResult, and MicrosoftOAuthCallbackOutcome into one file per type under src/Application/Authentication/ and delete the legacy file.
• Keep the lib/Common/namespace.php bootstrap in Web/microsoft-auth.php alongside vendor/autoload.php so Log's dependencies and the global exception handler keep working; dropping it caused OAuth error callbacks to fatal with "Class 'Configuration' not found" in an earlier attempt (PR Adding PSR-4 support #1552).
• Add an entry-point regression test that runs Web/microsoft-auth.php in a subprocess to catch missing-class fatals in the bootstrap.
• Map LibreBooking\Tests\ to tests/src/ via autoload-dev and move the handler test there as a namespaced class; the strict CI autoload check now enforces that test names match their paths. Add a "src" PHPUnit testsuite for running these tests in isolation.
• Add src to both PHPStan configurations.
• Document the migration policy in AGENTS.md: what belongs in src/, the one-type-per-file and namespace rules, the test location and naming convention, and the entry-point bootstrap requirements.

Related: #1501

Assisted-by: Claude:claude-fable-5

[Copilot]

Pull request overview

This PR establishes initial PSR-4 autoloading for the LibreBooking\ namespace via Composer, introduces the first migrated src/ component (Microsoft OAuth callback handler and related types), and adds a subprocess regression test to ensure the web entry point can execute without missing-class fatals while still relying on the legacy bootstrap.

Changes:

• Add Composer PSR-4 autoload (LibreBooking\ → src/) and autoload-dev mapping for LibreBooking\Tests\ → tests/src/.
• Migrate Microsoft OAuth callback handler/types into src/Application/Authentication/ with one-type-per-file and update the web entry point to use Composer autoloading.
• Add a subprocess test for Web/microsoft-auth.php and a dedicated PHPUnit src testsuite; include src/ in PHPStan configs; document migration rules in AGENTS.md.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Web/microsoft-auth.php	Switch callback handler loading to Composer autoload while retaining legacy bootstrap.
tests/Web/MicrosoftAuthEntryPointTest.php	Add subprocess regression test to catch entry-point bootstrap fatals.
tests/src/Application/Authentication/MicrosoftOAuthCallbackTest.php	Move handler unit tests under PSR-4 test namespace/path.
src/Application/Authentication/MicrosoftOAuthCallback.php	Introduce namespaced handler under src/ with log-sanitization.
src/Application/Authentication/MicrosoftOAuthCallbackOutcome.php	Split outcome enum into its own PSR-4-compliant file.
src/Application/Authentication/MicrosoftOAuthCallbackResult.php	Split result DTO into its own PSR-4-compliant file.
phpunit.xml.dist	Add src testsuite for isolating PSR-4 tests.
phpstan.neon	Add src/ to PHPStan analysis paths.
phpstan_next.neon	Add src/ to PHPStan-next analysis paths.
composer.json	Add PSR-4 autoload/autoload-dev mappings for src/ and tests/src/.
AGENTS.md	Document PSR-4 migration policy, placement rules, and testing conventions.

[barakiva]

@JohnVillalovos thank you for the help!