Fix execCreate() to send numeric $user as string

[clue]

This changeset fixes execCreate() to send a numeric $user as string. The $user parameter accepts a string|int, but a numeric user ID such as 1000 was sent as a JSON number, which Docker rejects with 400 Bad Request. This has been broken since the parameter was added in #39 (2016), passing the user ID as a string such as '1000' worked as a workaround.

Builds on top of #39

[Copilot]

🟢 Approval recommended

The focused fix preserves string inputs and includes targeted regression coverage, with no unresolved issues.

No open issues 🎉

What changed in this PR

Fixes execCreate() so integer user IDs are serialized as strings, avoiding Docker’s 400 Bad Request response.

Changes:

• Casts $user to a string before JSON serialization.
• Adds a regression test verifying integer 1000 is sent as "1000".

File	Description
tests/​ClientTest.php	Adds numeric user ID regression coverage.
src/​Client.php	Serializes the exec user as a string.

_{Review effort: Balanced}