fix(#587): forward session_id on image.attach_bytes and await the RPC (#588)

* fix(#587): forward session_id on image.attach_bytes and await the RPC

The mobile image-send path called image.attach_bytes without the required
session_id field, so the gateway resolver (_sess) returned
JsonRpcError(code=4001, message=session not found).

- Add "session_id" to agentSessionId to the image.attach_bytes params
  (desktop reference client already does this; mobile was the outlier).
- Switch the image branch from fire-and-forget wsClient.send to the
  awaited sendRpcAndAwait() layer (same awaited-RPC path used for
  slash commands, #526). This also removes the race where prompt.submit
  could fire before the image was staged, and surfaces RPC errors
  instead of silently swallowing them.

Root cause verified against tui_gateway/server.py _sess() and the
desktop use-prompt-actions reference. ChatViewModel.kt passes ktlintCheck.

Closes #587

* fix(#587): also forward session_id on file.attach (same 4001 bug)

The image branch was fixed in the prior commit, but the file branch
(sendMessage else -> file.attach) omitted session_id too, so non-image
file uploads also hit JsonRpcError(code=4001, "session not found").
The backend file.attach handler (tui_gateway/server.py _sess) requires
session_id exactly like image.attach_bytes.

Add "session_id" to agentSessionId to the file.attach params, and
refresh the sendMessage flow comment (no longer fire-and-forget).

Verified by source: both image.attach_bytes and file.attach call the
same _sess resolver. Camera capture is unaffected (it produces an
image/jpeg attachment, which already routes through the fixed
image.attach_bytes branch).

ChatViewModel.kt passes ktlintCheck.

* test(#587): regression for image.attach_bytes session_id

Adds testSendMessage_imageAttachment_sendsSessionId, which proves the
image-attach RPC is issued with session_id (the missing field that
caused JsonRpcError 4001 'session not found').

Root blocker found: readContentUriBase64 constructs
android.util.Base64OutputStream, which throws MockK's 'Stub!' on the
JVM test, making the read return null and the image branch never run.
Mocking the Base64OutputStream constructor lets the read succeed so the
request(any(), capture(paramsSlot), any()) stub captures the params and
the session_id assertion holds.

Full ChatViewModelTest suite passes; ktlint-clean.

---------

Co-authored-by: M57 <m57@hermes-control.app>

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

@@ -642,8 +642,8 @@ class ChatViewModel(
      * Flow:
      * 1. Snapshot pending attachments (then clear them from UI)
      * 2. Add user message to UI + DB immediately (optimistic UX)
-     * 3. For each image → fire-and-forget `image.attach_bytes` (queues into session)
-     * 4. For each file → await `file.attach`, collect @file: refs
+     * 3. For each image → await `image.attach_bytes` (requires session_id)
+     * 4. For each file → await `file.attach` (requires session_id), collect @file: refs
      * 5. Send `prompt.submit` with text + @file: refs — images auto-picked up by backend
      */
     fun sendMessage(text: String) {
@@ -694,23 +694,38 @@ class ChatViewModel(
 
                 try {
                     if (attachment.isImage) {
-                        // Fire-and-forget — backend queues into session["attached_images"],
-                        // auto-picked by the subsequent prompt.submit
-                        wsClient.send(
-                            method = WsMethods.IMAGE_ATTACH_BYTES,
-                            params =
-                                mapOf(
-                                    "content_base64" to "data:${attachment.mimeType};base64,$b64",
-                                    "filename" to attachment.name,
-                                    "ext" to attachment.fileExtension,
-                                ),
-                        )
+                        // Await so the backend stages the image into
+                        // session["attached_images"] BEFORE prompt.submit runs
+                        // (a fire-and-forget send raced prompt.submit and the
+                        // image was dropped). Requires session_id or the gateway
+                        // 4001s "session not found" (desktop passes it too).
+                        val result =
+                            sendRpcAndAwait(
+                                method = WsMethods.IMAGE_ATTACH_BYTES,
+                                params =
+                                    mapOf(
+                                        "session_id" to agentSessionId,
+                                        "content_base64" to "data:${attachment.mimeType};base64,$b64",
+                                        "filename" to attachment.name,
+                                        "ext" to attachment.fileExtension,
+                                    ),
+                            )
+                        if (result != null) {
+                            @Suppress("UNCHECKED_CAST")
+                            val ok = (result as? Map<String, Any?>)?.get("attached") as? Boolean
+                            if (ok != true) {
+                                Log.w(TAG, "Image attach for ${attachment.name} returned non-ok: $result")
+                            }
+                        }
                     } else {
-                        // Await the @file: ref text so we can embed it in the prompt
+                        // Await the @file: ref text so we can embed it in the prompt.
+                        // file.attach also requires session_id or the gateway 4001s
+                        // "session not found" (same resolver as image.attach_bytes).
                         sendRpcAndAwait(
                             method = WsMethods.FILE_ATTACH,
                             params =
                                 mapOf(
+                                    "session_id" to agentSessionId,
                                     "data_url" to "data:${attachment.mimeType};base64,$b64",
                                     "name" to attachment.name,
                                 ),