[PHP] Always prefer host config over static servers from OpenAPI declaration

[stollr]

This PR fixes the bug described in #23016.

Currently the host in the config object is ignored if an operation declares its own servers in the OpenAPI file, which makes it impossible to override it in the application.

With this change, the host, defined in the config, always has priority over the servers declared in the operation.

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Summary by cubic

Always use the config host for PHP PSR-18 clients, even when an operation defines its own servers, so apps can override OpenAPI-declared hosts. Fixes #23016.

• Bug Fixes
  • Use config host if set; only fall back to operation servers when config host is empty, with hostIndex validation.

^{Written for commit 195abcf. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

[stollr]

After looking over the api class behaviour regarding the default hosts config once again, I realize that my solution could be problematic if specific operations have different server declarations than the rest and would lead to backward incompatibility.

So if anybody has an idea to improve this, you are welcome.