feat: Fix PHP client to avoid API mode values to silently fallback to…

[Benjamin-Freoua-Alma]

Reason for change

Linear task

Especially in a case sensitive context where :
mode = 'live' => Prod
mode = 'Live' => Sandbox
=> We should not have silent fallback.
Either a fallback with a warning : "Unknown value, falling back to sandbox"
or no fallback with an exception thrown.

Code changes

1. Exception thrown instead of silent fallback — the behavior is explicit.

2. True as the third parameter of in_array — strict comparison (type + case), which strengthens the detection of cases like 'Live' or 'TEST'.

3. The testInvalidMode test: the old assertion checked for a silent fallback to LIVE_MODE. It now tests that new Environment('INVALID_MODE') correctly throws an InvalidArgumentException with the correct message.

How to test

As a reviewer, you are encouraged to test the PR locally.

Checklist for authors and reviewers

• The title of the PR uses business wording, not technical jargon, for the changelog readers to understand it
• The PR implements the changes asked in the referenced task / issue
• The automated tests are compliant with the testing strategy
• The tests are relevant, and cover the corner/error cases, not only the happy path
• You understand the impact of this PR on existing code/features
• The changes include adequate logging and Datadog traces
• Documentation is updated (API, developer documentation, ADR, Notion...)

Non applicable