feat(snapshots): Emit diff_threshold in snapshot sidecar (EME-1055)

The generated Paparazzi test now reflects on each @Preview to read an
optional @SentrySnapshot(diffThreshold) annotation and writes the value
as "diff_threshold" in the per-snapshot sidecar JSON, but only when the
annotation is present and the threshold differs from the 0f default.

Changes SentrySnapshot's retention from BINARY to RUNTIME so the JVM
retains it for reflection. Java's Method.getAnnotation cannot see
CLASS/BINARY-retained annotations.

The lookup is guarded by runCatching so a missing runtime jar or a
class-loading failure degrades to "field omitted" rather than a test
failure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: runningcode

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Features
 
 - Add `@SentrySnapshot` runtime annotation for configuring the per-snapshot diff threshold, published as `io.sentry:sentry-snapshots-runtime` ([EME-1055](https://linear.app/getsentry/issue/EME-1055))
+- Emit `diff_threshold` in the snapshot sidecar JSON when a `@Preview` is also annotated with `@SentrySnapshot(diffThreshold = ...)` ([EME-1055](https://linear.app/getsentry/issue/EME-1055))
 
 ### Dependencies
 

plugin-build/src/main/kotlin/io/sentry/android/gradle/snapshot/GenerateSnapshotTestsTask.kt

@@ -376,6 +376,17 @@ class $CLASS_NAME(
         if (info.showSystemUi) metadata["showSystemUi"] = true
         if (info.showBackground) metadata["showBackground"] = true
 
+        val diffThreshold: Float? = runCatching {
+            val declaring = Class.forName(preview.declaringClass)
+            val method = declaring.declaredMethods.firstOrNull { it.name == preview.methodName }
+                ?: return@runCatching null
+            @Suppress("UNCHECKED_CAST")
+            val annClass = Class.forName("io.sentry.snapshots.runtime.SentrySnapshot") as Class<out Annotation>
+            val ann = method.getAnnotation(annClass) ?: return@runCatching null
+            annClass.getDeclaredMethod("diffThreshold").invoke(ann) as? Float
+        }.getOrNull()
+        if (diffThreshold != null && diffThreshold != 0f) metadata["diff_threshold"] = diffThreshold
+
         val json = metadata.entries.joinToString(",\n  ", prefix = "{\n  ", postfix = "\n}") { (k, v) ->
             if (v is String) "\"" + k + "\": \"" + escapeJson(v) + "\""
             else "\"" + k + "\": " + v

plugin-build/src/test/kotlin/io/sentry/android/gradle/snapshot/GenerateSnapshotTestsTaskTest.kt

@@ -126,6 +126,26 @@ class GenerateSnapshotTestsTaskTest {
     )
   }
 
+  @Test
+  fun `generated sidecar reads SentrySnapshot annotation via reflection`() {
+    val content = generateAndRead(packageTrees = listOf("com.example"))
+
+    assertTrue(content.contains("Class.forName(preview.declaringClass)"))
+    assertTrue(content.contains("\"io.sentry.snapshots.runtime.SentrySnapshot\""))
+    assertTrue(content.contains("getDeclaredMethod(\"diffThreshold\")"))
+  }
+
+  @Test
+  fun `generated sidecar emits diff_threshold only when non-default`() {
+    val content = generateAndRead(packageTrees = listOf("com.example"))
+
+    assertTrue(
+      content.contains(
+        "if (diffThreshold != null && diffThreshold != 0f) metadata[\"diff_threshold\"] = diffThreshold"
+      )
+    )
+  }
+
   @Test
   fun `parseMajorVersion extracts major from standard semver`() {
     assertEquals(1, parseMajorVersion("1.3.5"))

sentry-snapshots-runtime/src/main/kotlin/io/sentry/snapshots/runtime/SentrySnapshot.kt

@@ -8,7 +8,7 @@ import androidx.annotation.FloatRange
  * Apply alongside `@Preview` on a composable to override the global diff threshold on a
  * per-snapshot basis.
  */
-@Retention(AnnotationRetention.BINARY)
+@Retention(AnnotationRetention.RUNTIME)
 @Target(AnnotationTarget.FUNCTION)
 annotation class SentrySnapshot(
   /**