feat: Make SSE endpoint configurable in server transports

This commit updates the WebFluxSseServerTransport, WebMvcSseServerTransport, and HttpServletSseServerTransport classes to accept the SSE endpoint path as a constructor argument.

This change provides more flexibility in configuring the SSE endpoint, allowing it to be customized instead of being hardcoded to "/sse".
It also aligns the Java SDK's behavior with the Python SDK's SseServerTransport, which already supports this customization.

A default SSE endpoint of "/sse" is maintained for backward compatibility.

Also, addressed minor code quality issues identified by SonarLint and reordered modifiers for static final fields to conform with the Java Language Specification.

Signed-off-by: Alexandros Pappas <alexandros.pappas@yiluhub.com>