test: add JaCoCo coverage + 83% line gate, raise coverage to ~85%

The SDK had no coverage instrumentation. The existing SdkTest aborts at
@BeforeAll without a live Firefox, so almost no logic was measured (~33%
floor). This adds JaCoCo (prepare-agent + report + check) and a new
mock/HTTP-based PercyLogicTest (43 tests) that covers the SDK logic
without a browser, lifting non-driver line coverage to ~84.7% (the CI
browser run reaches ~87.8%).

Gate: jacoco:check bound to the test phase, LINE minimum 0.83 — the
honest floor for the unit-testable logic. The remaining uncovered lines
are live-WebDriver-only (the JS-execution/cookie body of snapshot(),
changeWindowDimensionAndWait CDP/resize, processFrame iframe recovery,
the responsive-capture sleep) and are exercised by SdkTest on the
browser CI (MOZ_HEADLESS=1 npx percy exec -- mvn test).

PercyLogicTest covers: createRegion variants, snapshot/screenshot
dispatch + guards, web DOM post + automate session metadata + region
element conversion, responsive width-config HTTP, readiness config +
waitForReady disabled paths, getSerializedDOM (cookies, cross-origin
iframe, readiness diagnostics), healthcheck branches, log, Environment,
setClientInfo, DriverMetadata + Cache.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: AkashBrowserStack

pom.xml

@@ -184,6 +184,51 @@
           <useSystemClassLoader>false</useSystemClassLoader>
         </configuration>
       </plugin>
+      <plugin>
+        <groupId>org.jacoco</groupId>
+        <artifactId>jacoco-maven-plugin</artifactId>
+        <version>0.8.12</version>
+        <executions>
+          <execution>
+            <id>prepare-agent</id>
+            <goals>
+              <goal>prepare-agent</goal>
+            </goals>
+          </execution>
+          <execution>
+            <id>report</id>
+            <phase>test</phase>
+            <goals>
+              <goal>report</goal>
+            </goals>
+          </execution>
+          <execution>
+            <id>check</id>
+            <phase>test</phase>
+            <goals>
+              <goal>check</goal>
+            </goals>
+            <configuration>
+              <rules>
+                <rule>
+                  <element>BUNDLE</element>
+                  <limits>
+                    <limit>
+                      <counter>LINE</counter>
+                      <value>COVEREDRATIO</value>
+                      <!-- Honest floor for the unit-testable (non-driver) logic.
+                           The live-WebDriver paths in SdkTest add headroom to
+                           ~0.88 on the browser CI, but the deterministic
+                           mock/HTTP-covered logic alone clears 0.83. -->
+                      <minimum>0.83</minimum>
+                    </limit>
+                  </limits>
+                </rule>
+              </rules>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
       <plugin>
         <artifactId>maven-jar-plugin</artifactId>
         <version>3.3.0</version>