chore: add php v2 transition server

[josecorella]

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Description of changes:

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[Copilot]

Pull Request Overview

This PR adds a new PHP v2 transition server to support transitional encryption client testing, specifically for handling both V2 and legacy encryption modes.

• Adds complete PHP v2 transition server implementation with client management, object storage operations, and session handling
• Integrates the new server into the existing test infrastructure and CI pipeline
• Updates git submodules and test configurations to support the transition server functionality

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test-server/php-v2-transition-server/src/*.php	Core server implementation files for handling HTTP routing, S3 operations, client management, and error responses
test-server/php-v2-transition-server/composer.json	PHP dependency configuration and project metadata
test-server/php-v2-transition-server/Makefile	Build and server management commands
test-server/php-v2-transition-server/.duvet/config.toml	Code coverage and specification compliance configuration
test-server/java-tests/src/it/java/software/amazon/encryption/s3/TestUtils.java	Integration of PHP v2 transition server into test framework
.gitmodules	Git submodule configuration for PHP SDK dependency
.github/workflows/test.yml	CI pipeline updates to include PHP v2 transition server setup

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