Improve OAuth token URL configuration errors - #2245

Fixes #2179

Adds validation for configured OAuth token endpoint URLs so invalid values fail with a clear ConfigurationException.
Changes:

• Validate micronaut.security.oauth2.clients.*.token.url when building the token endpoint.
• Require an absolute http or https URL with a valid host.
• Include the OAuth client name, property path, and configured value in the error message.
• Add regression coverage for missing schemes, malformed HTTP URLs, relative paths, and unsupported schemes.