fix(chat): dedupe user bubbles enriched with attachment ref lines (#952)

Sending an image/file renders the user message twice: the optimistic
bubble carries the raw caption while the backend persists an enriched
copy with "@image:" / "@file:" ref lines and a "[screenshot]" marker
(_build_image_ref_message / run_agent multipart flattening). The trimmed
exact-content compare in sameLogicalMessage fails on that drift and the
prefix fallback gate (>=40 chars) rejects short captions, so the REST
transcript sync appends the REST copy as a second bubble.

Compare USER rows on the caption with attachment-injection lines
stripped (stripAttachmentRefLines). USER-only so assistant text that
legitimately mentions these tokens is never collapsed.

Device-verified on the hermes37 emulator: image send + transcript
resync renders a single user bubble, restart/reload stays clean.
Unit: ChatToolDedupeTest 32/32.

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatViewModel.kt

@@ -141,6 +141,20 @@ internal fun sameLogicalMessage(
     val ta = a.content.trim()
     val tb = b.content.trim()
     if (ta == tb) return true
+    // Attachment dedupe: when the user sends an image/file, the backend
+    // persists an ENRICHED copy of the prompt — the real caption plus
+    // `@image:`/`@file:` reference lines and a `[screenshot]` marker
+    // (tui_gateway _build_image_ref_message / run_agent flattening). The
+    // optimistic bubble carries the raw caption only, so the exact compare
+    // above fails and the REST sync renders the same message twice. Compare
+    // USER rows on the caption with injection lines stripped (both sides —
+    // stripping a clean string is a no-op). USER-only so assistant text that
+    // legitimately mentions these tokens is never collapsed.
+    if (a.role == MessageRole.USER) {
+        val ca = stripAttachmentRefLines(ta)
+        val cb = stripAttachmentRefLines(tb)
+        if (ca == cb) return true
+    }
     // Issue #842: a seal race can leave the live orphan a few trailing tokens
     // short of the streamed narration (the last delta was still in the
     // throttled buffer when tool.start sealed the message). The backend
@@ -153,6 +167,25 @@ internal fun sameLogicalMessage(
         (tb.startsWith(ta) || ta.startsWith(tb))
 }
 
+/**
+ * Remove backend attachment-injection lines from a user message so the
+ * optimistic bubble and the server-enriched REST copy compare equal.
+ * Lines the gateway adds on top of the user's caption: `@image:<path>`,
+ * `@file:<ref>`, and `[screenshot]` (multipart image placeholder written by
+ * run_agent). Blank lines left behind are dropped too.
+ */
+internal fun stripAttachmentRefLines(content: String): String =
+    content
+        .lines()
+        .map { it.trim() }
+        .filterNot { line ->
+            line.startsWith("@image:") ||
+                line.startsWith("@file:") ||
+                line == "[screenshot]"
+        }
+        .joinToString("\n")
+        .trim()
+
 /**
  * Room accumulates BOTH the WS-persisted copy (UUID id, rich tool payload,
  * tool name) and the REST-persisted copy (`rest-` id, result-only payload,

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

@@ -133,6 +133,50 @@ class ChatToolDedupeTest {
         assertFalse(sameLogicalMessage(ws, rest))
     }
 
+    @Test
+    fun sameLogicalMessage_userAttachmentEnrichment_stillSame() {
+        // User attaches an image: the optimistic bubble carries the RAW caption,
+        // the backend persists caption + `@image:` ref + `[screenshot]` marker.
+        // They are the same logical message — the sync must not duplicate it.
+        val optimistic = ChatMessage(role = MessageRole.USER, content = "what is this image")
+        val rest =
+            ChatMessage(
+                role = MessageRole.USER,
+                content =
+                    "what is this image\n" +
+                        "@image:/.hermes/images/upload_20260822_111444_1.jpg\n" +
+                        "[screenshot]",
+            )
+        assertTrue(sameLogicalMessage(optimistic, rest))
+        assertTrue(sameLogicalMessage(rest, optimistic))
+    }
+
+    @Test
+    fun sameLogicalMessage_userFileRefEnrichment_stillSame() {
+        // Same drift for file attachments: REST gains a `@file:` ref line.
+        val optimistic = ChatMessage(role = MessageRole.USER, content = "summarize this doc")
+        val rest = ChatMessage(role = MessageRole.USER, content = "summarize this doc\n@file:report.pdf")
+        assertTrue(sameLogicalMessage(optimistic, rest))
+    }
+
+    @Test
+    fun sameLogicalMessage_userDifferentCaptionsWithAttachments_notSame() {
+        // Stripping must not over-collapse: two DIFFERENT user captions that
+        // both carried images stay distinct.
+        val a = ChatMessage(role = MessageRole.USER, content = "look at this\n@image:/a.jpg")
+        val b = ChatMessage(role = MessageRole.USER, content = "now this one\n@image:/b.jpg")
+        assertFalse(sameLogicalMessage(a, b))
+    }
+
+    @Test
+    fun sameLogicalMessage_assistantMentioningRefLines_notStripped() {
+        // The strip is USER-only: an assistant reply that literally contains
+        // an @image: line must never merge with a different assistant message.
+        val a = ChatMessage(role = MessageRole.ASSISTANT, content = "here you go\n@image:/x.jpg")
+        val b = ChatMessage(role = MessageRole.ASSISTANT, content = "here you go")
+        assertFalse(sameLogicalMessage(a, b))
+    }
+
     // ── dedupeCachedMessages ───────────────────────────────────────────────
 
     @Test