fix(terminal): report unavailable only when no serializer answered

The previous commit reported `unavailable: no-serializable-buffer` whenever the
serializer's answer was empty. Clients map that reason to retry-worthy and banner
after the seventh such answer, so a genuinely empty pane -- a fresh one that has
produced no output -- would retry and then show a loss banner. That is the case
`accepts an empty snapshot as successful recovery without a loss banner` exists to
prevent; it kept passing only because it stubs the transport and never sees what the
host now sends. #14095 makes the client request a tagged restore on every recovery
whose pushed snapshot was empty, so this path is routine rather than rare.

`unavailable` means nobody answered. A serializer that answered with an empty buffer
has answered. Restrict the reason to a null serializer on both paths, and keep the
empty-content check for the payload and for seq/source metadata, which is the part
that must not be published from an empty snapshot.

src/main/runtime/rpc/methods/terminal.ts

@@ -2352,8 +2352,11 @@ export const TERMINAL_METHODS: RpcAnyMethod[] = [
             pendingEscapeTailAnsi: serializedContent?.pendingEscapeTailAnsi,
             truncated: false,
             truncatedByByteBudget: serialized?.truncatedByByteBudget,
-            // Why: an absent or empty serializer answer is not proof the pane is empty.
-            unavailable: serializedContent ? undefined : 'no-serializable-buffer',
+            // Why: `unavailable` means nobody answered, so the client retries and can
+            // eventually banner. A serializer that DID answer with an empty buffer has
+            // answered — reporting that as unavailable banners genuinely empty panes.
+            // Empty content still suppresses the seq/source metadata below.
+            unavailable: serialized ? undefined : 'no-serializable-buffer',
             data: serializedContent?.data ?? ''
           })
         } catch (error) {
@@ -2695,8 +2698,11 @@ export const TERMINAL_METHODS: RpcAnyMethod[] = [
               kittyKeyboardFlags: serializedContent?.kittyKeyboardFlags,
               oscLinks: serializedContent?.oscLinks,
               pendingEscapeTailAnsi: serializedContent?.pendingEscapeTailAnsi,
+              // Why: same rule as the tagged path — only an absent serializer answer is
+              // "unavailable". A serializer that answered empty, with an empty retained
+              // tail, is a genuinely empty pane and must not be reported as a failure.
               unavailable:
-                serializedContent || read.tail.length > 0 ? undefined : 'no-serializable-buffer',
+                serialized || read.tail.length > 0 ? undefined : 'no-serializable-buffer',
               data: serializedContent?.data ?? retainedTail
             }
           )

src/main/runtime/rpc/terminal-requested-snapshot-unavailability.test.ts

@@ -168,14 +168,28 @@ describe('requested terminal snapshot unavailability reasons', () => {
     expect(start.unavailable).toBeUndefined()
   })
 
-  it('reports no-serializable-buffer for an empty snapshot object', async () => {
+  // Why: `unavailable` drives client retries and eventually a loss banner. A serializer
+  // that answered with an empty buffer HAS answered; a genuinely empty pane must not be
+  // reported as a failure or every fresh pane banners. Only an absent answer is a failure.
+  it('does not report a reason when the serializer answered with an empty buffer', async () => {
     const { start, chunks } = await requestSnapshotReply({
       connectionId: 'conn-reason-empty',
       serializeRequested: async () => ({ data: '', cols: 120, rows: 40 })
     })
     expect(chunks).toBe('')
     expect(start).toMatchObject({ requestId: 77, truncated: false })
-    expect(start.unavailable).toBe('no-serializable-buffer')
+    expect(start.unavailable).toBeUndefined()
+  })
+
+  // Why: an empty answer must still not publish seq/source metadata, or the client
+  // anchors its stream to a snapshot that carries no content.
+  it('omits seq and source metadata for an empty serialized answer', async () => {
+    const { start } = await requestSnapshotReply({
+      connectionId: 'conn-reason-empty-metadata',
+      serializeRequested: async () => ({ data: '', cols: 120, rows: 40 })
+    })
+    expect(start.seq).toBeUndefined()
+    expect(start.source).toBeUndefined()
   })
 
   it('accepts scrollback-only serialized content', async () => {