Fix the baseUrl is configured with a trailing slash #645

[leehaut]

Fixes #645

Replace unsafe string concatenation with UriComponentsBuilder to build a normalized, valid URL:

UriComponentsBuilder.fromHttpUrl(baseUrl)
    .path(messageEndpoint)
    .queryParam("sessionId", sessionId)
    .build()
    .toUriString();

Testing

• Verified with baseUrl=http://localhost:8080/ → correct URL
• Verified with baseUrl=http://localhost:8080 → correct URL
• Integration test added to assert URL format