PHP SDK: add BDD scenarios and bdd-php component

[countradooku]

Follow-up from the experimental PHP SDK review.

Every other SDK has BDD coverage wired through repository component detection, but the PHP SDK does not yet have bdd/php/ scenarios or a bdd-php component entry. This is fine as a follow-up and not a merge blocker, but should be tracked so it does not get lost.

Scope

• Add a bdd/php/ test suite following the shape of the other SDK BDD directories.
• Add a bdd-php component entry in .github/config/components.yml.
• Wire the component paths so PHP BDD changes trigger the expected CI tasks.
• Ensure the PHP SDK component dependencies include the right Rust SDK/server/CI infrastructure dependencies for BDD execution.

References

• bdd/python/, bdd/node/, bdd/go/, bdd/java/, bdd/csharp/, bdd/rust/
• .github/config/components.yml

Related: #3302
Initial PHP SDK work: #3235