Establish PHP 8.3 development baseline and split unit vs. integration infrastructure

[Copilot]

This PR establishes the v3 development baseline for the client: modern Composer metadata, isolated unit/integration test configuration, static analysis and style tooling, and a CI workflow that separates default quality checks from containerized integration execution.

• Tooling baseline

  • raises the package requirement to PHP 8.3
  • narrows runtime dependencies to PSR interfaces (psr/http-client, psr/http-factory, psr/log)
  • adds the requested dev tooling (phpunit, mockery, phpstan, phpcs, guzzle, league/openapi-psr7-validator)
  • adds Composer scripts for the default quality gates

• Test and analysis configuration

  • splits PHPUnit into unit and integration suites
  • excludes integration and testcontainers groups by default
  • adds a max-level PHPStan config for src/
  • adds a PHPStan baseline so the new gate is usable against the current codebase without widening scope into a full typing rewrite

• Bootstrap and autoload discovery

  • updates test/bootstrap.php to discover Composer autoloaders first
  • adds fallback loading for system-installed packages and project classes when Composer autoload is unavailable
  • keeps test bootstrap usable in both local and CI environments

• CI pipeline

  • replaces the previous monolithic workflow with:
    • a default unit job for push / pull_request
    • a testcontainers job gated behind workflow_dispatch
  • adds explicit minimal workflow permissions
  • keeps Docker-based integration execution isolated from the default feedback loop

• Codebase hygiene for v3 files

  • adds declare(strict_types=1); across PHP sources, tests, and examples
  • removes closing tags
  • normalizes src/ to PSR-12 expectations
  • marks resource-backed tests as integration-scoped so unit runs stay fast and deterministic

Example of the new PHPUnit split:

<testsuites>
    <testsuite name="unit">
        <directory>test/ZammadAPIClient</directory>
        <exclude>test/ZammadAPIClient/Resource</exclude>
    </testsuite>
    <testsuite name="integration">
        <directory>test/ZammadAPIClient/Resource</directory>
    </testsuite>
</testsuites>

<groups>
    <exclude>
        <group>integration</group>
        <group>testcontainers</group>
    </exclude>
</groups>