Validate Spring AI version update arguments

[codingkiddo]

Summary

Improve scripts/update-spring-ai-version.sh argument validation.

Previously, running the script without a version silently defaulted to 2.0.0-RC1, which could unintentionally rewrite many project POM files.

This change:

• requires exactly one Spring AI version argument;
• rejects missing and extra arguments with a usage message;
• preserves existing invalid-version validation;
• adds regression tests for missing, extra, and invalid arguments.

Testing

• Missing version returns exit code 2.
• Extra arguments return exit code 2.
• Invalid version returns exit code 1.
• No POM files are modified when validation fails.