fix(chat): context meter flashes stale window on mid-chat model swap (#817) (#846)

* fix(chat): meter flashes stale window on mid-chat model swap (issue #817)

session.info updates the model label as soon as the backend applies a
swap, but the context meter's denominator kept the old model's window
until the next fetch — the chip briefly rendered the new label over the
old window (e.g. 1M flashing before settling on 262k).

- session.info with a DIFFERENT model now blanks the denominator (chip
  hides while resolving) and immediately re-fires fetchContextUsage()
- fetchContextUsage writes the denominator ONCE, atomically: RPC
  context_max wins, REST model/info is the fallback — a stale pre-swap
  response can never override a fresh one
- regression tests: swap re-fires the fetch and lands the new window;
  an identical session.info leaves the meter untouched

* fix(chat): don't fall back to profile REST window on model swap (issue #817)

After a session-scoped swap the live agent's context_breakdown can
return empty (context_max 0) while the runtime warms up. The refetch
then fell back to GET /api/model/info — PROFILE-scoped — which reports
the profile's main-model window (e.g. 1M), resurrecting the stale
window under the new label. The swap-triggered refetch now trusts only
the RPC's live context_max; the chip stays hidden until the real
window lands.

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/test/java/com/m57/hermescontrol/ui/chat/ChatViewModelTest.kt

@@ -1906,6 +1906,100 @@ class ChatViewModelTest {
             }
         }
 
+    @Test
+    fun testSessionInfoModelSwap_blanksStaleMeterAndRefetches() =
+        runTest {
+            stubEmptySessionRests("session-a")
+            val (viewModel, _) = createViewModelWithSession()
+            advanceUntilIdle()
+
+            // Establish the OLD model's label through the real event path.
+            mockEventsFlow.emit(
+                WsEvent.SessionInfo(
+                    mapOf("model" to "deepseek-v4-flash", "provider" to "opencode-go"),
+                ),
+            )
+            advanceUntilIdle()
+            assertEquals("opencode-go/deepseek-v4-flash", viewModel.uiState.value.currentSessionModel)
+
+            // Fetch the meter so it carries the OLD model's window (1M).
+            var contextMax = 1_000_000L
+            var breakdownCalls = 0
+            every { HermesWsClient.request(WsMethods.SESSION_CONTEXT_BREAKDOWN, any(), any()) } answers {
+                breakdownCalls++
+                CompletableDeferred<Any?>(
+                    mapOf("context_max" to contextMax, "context_used" to 42025L),
+                )
+            }
+            viewModel.fetchContextUsage()
+            advanceUntilIdle()
+            assertEquals(1_000_000L, viewModel.uiState.value.fullContextTokens)
+
+            // The swap: the live agent hasn't warmed up yet, so the RPC comes
+            // back empty — the meter must NOT fall back to the profile-scoped
+            // REST window (which describes the OLD model). It stays hidden.
+            contextMax = 0L
+            val callsBeforeSwap = breakdownCalls
+            mockEventsFlow.emit(
+                WsEvent.SessionInfo(
+                    mapOf("model" to "tencent/hy3:free", "provider" to "nous"),
+                ),
+            )
+            advanceUntilIdle()
+
+            assertEquals("nous/tencent/hy3:free", viewModel.uiState.value.currentSessionModel)
+            assertTrue("model swap must re-fire the meter fetch", breakdownCalls > callsBeforeSwap)
+            assertNull(
+                "cold RPC after a swap must keep the meter hidden, not show the old window",
+                viewModel.uiState.value.fullContextTokens,
+            )
+
+            // The runtime warms up — the next fetch lands the NEW model's window.
+            contextMax = 262_144L
+            viewModel.fetchContextUsage()
+            advanceUntilIdle()
+            assertEquals(262_144L, viewModel.uiState.value.fullContextTokens)
+        }
+
+    @Test
+    fun testSessionInfoSameModel_keepsMeterUntouched() =
+        runTest {
+            stubEmptySessionRests("session-a")
+            val (viewModel, _) = createViewModelWithSession()
+            advanceUntilIdle()
+
+            // Establish label + meter through the real paths.
+            mockEventsFlow.emit(
+                WsEvent.SessionInfo(
+                    mapOf("model" to "tencent/hy3:free", "provider" to "nous"),
+                ),
+            )
+            advanceUntilIdle()
+            var breakdownCalls = 0
+            every { HermesWsClient.request(WsMethods.SESSION_CONTEXT_BREAKDOWN, any(), any()) } answers {
+                breakdownCalls++
+                CompletableDeferred<Any?>(
+                    mapOf("context_max" to 262_144L),
+                )
+            }
+            viewModel.fetchContextUsage()
+            advanceUntilIdle()
+            assertEquals(262_144L, viewModel.uiState.value.fullContextTokens)
+            val callsBefore = breakdownCalls
+
+            // Identical model again — no swap, no blank, no refetch.
+            mockEventsFlow.emit(
+                WsEvent.SessionInfo(
+                    mapOf("model" to "tencent/hy3:free", "provider" to "nous"),
+                ),
+            )
+            advanceUntilIdle()
+
+            assertEquals(callsBefore, breakdownCalls)
+            assertEquals(262_144L, viewModel.uiState.value.fullContextTokens)
+            assertEquals("nous/tencent/hy3:free", viewModel.uiState.value.currentSessionModel)
+        }
+
     @Test
     fun testResumeNotFound_recoversWithNewSession() =
         runTest {