test: Add PHP image smoke checks

[musaabhasan]

Description

Adds a common PHP image smoke-test script and runs it during each PHP Dockerfile build. The check validates core runtime expectations that the Totara dev images rely on:

• required PHP extensions are loaded
• SQL Server extensions are present on supported PHP versions
• Composer is executable
• php-fpm configuration parses successfully
• required locales are generated

Refs #38

Testing Instructions

1. Check out this pull request
2. Build a PHP image, e.g. docker build -f php/php83/Dockerfile php
3. Confirm the build runs /usr/local/bin/totara-php-smoke-test successfully
4. Remove or disable one checked extension or locale and confirm the build fails during the smoke-test step

Local validation performed:

• bash -n php/smoke-test.sh
• git diff --check

Docker builds were not run locally because Docker is not installed on this workstation.

Checklist

• Does what the author says it will do
• Testing instructions are provided
• Commit messages make sense and follow the conventional commit standard
• No identified security issues
• No identified maintenance issues
• Any third-party libraries/dependencies use the MIT or Apache 2.0 license
• Changes made are backwards compatible and will not break existing setups
• Changes to scripts in the bin/ directory run correctly on both MacOS and WSL
• Changes to containers can be built locally sucessfully (e.g. via tbuild container && tup container)
• Containers/images are compatible with both AMD64 (Windows) and ARM64 (MacOS)
• Changes made to config.php are compatible with our oldest supported Totara version, our newest Totara version, and Moodle