Fix explorer back and session usage selection

app/androidApp/src/test/kotlin/com/example/aiapp/SessionUsageTest.kt

@@ -0,0 +1,90 @@
+package com.example.aiapp
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class SessionUsageTest {
+    @Test
+    fun `usage snapshots stay with the session's machine and provider`() {
+        val claude = snapshot("machine", "claude", null)
+        val codex = snapshot("machine", "codex", "codex")
+        val reserve = snapshot("machine", "codex", "gpt-reserve")
+        val elsewhere = snapshot("other", "codex", "codex")
+
+        assertEquals(
+            listOf(codex, reserve),
+            usageSnapshotsFor(
+                listOf(claude, codex, reserve, elsewhere),
+                setup = "machine",
+                provider = "codex",
+            ),
+        )
+    }
+
+    @Test
+    fun `a session without a meter has no usage snapshots`() {
+        assertEquals(
+            emptyList(),
+            usageSnapshotsFor(
+                listOf(snapshot("machine", "claude", null)),
+                setup = "machine",
+                provider = null,
+            ),
+        )
+    }
+
+    @Test
+    fun `the model selects its named pool and other models use the generic pool`() {
+        val generic = snapshot("machine", "codex", "codex")
+        val spark =
+            snapshot(
+                "machine",
+                "codex",
+                "codex_bengalfox",
+                limitName = "GPT-5.3-Codex-Spark",
+            )
+        val reserve =
+            snapshot("machine", "codex", "base_model_inference", limitName = "gpt-reserve")
+        val pools = listOf(generic, spark, reserve)
+
+        assertEquals(spark, usagePoolFor(pools, "gpt-5.3-codex-spark"))
+        assertEquals(reserve, usagePoolFor(pools, "gpt-5.6-luna"))
+        assertEquals(generic, usagePoolFor(pools, "gpt-6-astra"))
+    }
+
+    @Test
+    fun `the bar uses the shortest reported cycle`() {
+        val weekly = window("Weekly", 10_080)
+        val hourly = window("5-hour window", 300)
+
+        assertEquals(hourly, shortestUsageWindow(listOf(weekly, hourly)))
+        assertEquals(null, shortestUsageWindow(listOf(window("unknown", null))))
+    }
+
+    private fun snapshot(
+        setup: String,
+        provider: String,
+        limitId: String?,
+        limitName: String? = null,
+    ) =
+        UsageSnapshot(
+            provider = provider,
+            setup = setup,
+            setupName = setup,
+            limitId = limitId,
+            limitName = limitName,
+            state = "ok",
+            detail = null,
+            windows = emptyList(),
+        )
+
+    private fun window(label: String, durationMinutes: Long?) =
+        UsageWindow(
+            kind = "test",
+            label = label,
+            percent = 12.0,
+            durationMinutes = durationMinutes,
+            resetsAt = null,
+            active = false,
+        )
+}