Fix phpstan mixed type in service assertion

[ohmyfelix]

Summary

Fix PHPStan errors in McpExtension by making the schema assertion callback safely handle mixed input before calling string-only functions.

Motivation

Scheduled CI started failing in the Phpstan workflow because the callback passed to Expect::string()->assert() forwarded a mixed value directly to str_starts_with(), class_exists(), and interface_exists().

Changes

• update the service assertion callback in src/DI/McpExtension.php to guard with is_string($input)
• keep behavior unchanged for valid string service references and class/interface names

Testing

• make phpstan
• Full test suite
• CI passes

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