Add JaCoCo coverage and improve tests to 98.7%

Add JaCoCo plugin for coverage reporting. Add additional tests
for null/edge cases in TelnetSession, TelnetServer, and
WebgitApplication.main to maximize coverage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

build.gradle.kts

@@ -1,5 +1,6 @@
 plugins {
     java
+    jacoco
     id("org.springframework.boot") version "4.0.3"
     id("io.spring.dependency-management") version "1.1.7"
 }
@@ -41,4 +42,12 @@ dependencies {
 
 tasks.withType<Test> {
     useJUnitPlatform()
+    finalizedBy(tasks.jacocoTestReport)
+}
+
+tasks.jacocoTestReport {
+    dependsOn(tasks.test)
+    reports {
+        csv.required = true
+    }
 }