fix(slack): upload-file silently drops a caption (#121047)

The Slack `upload-file` action accepts `media` as an alias for `filePath`, so
a caller can reach it with the same vocabulary used for a media `send`. That
vocabulary carries its accompanying text in `caption`, but the upload path
read only `initialComment` and `message`, so the text was dropped and the file
arrived in the channel with no comment at all.

Accept `caption` as the lowest-precedence alias. Explicit `initialComment`
still wins, `message` still comes next, and an explicitly empty
higher-precedence value stays empty.

extensions/slack/src/message-action-dispatch.test.ts

@@ -738,6 +738,32 @@ describe("handleSlackMessageAction", () => {
         initialComment: "path alias",
       },
     },
+    {
+      name: "maps an upload-file caption to the upload's initial comment",
+      params: {
+        channelId: "C1",
+        media: "/tmp/chart.png",
+        caption: "chart attached",
+      },
+      expected: {
+        filePath: "/tmp/chart.png",
+        initialComment: "chart attached",
+      },
+    },
+    {
+      name: "prefers an explicit upload-file initial comment over message and caption",
+      params: {
+        channelId: "C1",
+        media: "/tmp/chart.png",
+        initialComment: "",
+        message: "message text",
+        caption: "caption text",
+      },
+      expected: {
+        filePath: "/tmp/chart.png",
+        initialComment: "",
+      },
+    },
   ])("$name", async ({ params, expected }) => {
     const invoke = createInvokeSpy();
     const cfg = slackConfig();

extensions/slack/src/message-action-dispatch.ts

@@ -358,6 +358,10 @@ export async function handleSlackMessageAction(params: {
         initialComment:
           readStringParam(actionParams, "initialComment", { allowEmpty: true }) ??
           readStringParam(actionParams, "message", { allowEmpty: true }) ??
+          // `media` is accepted as an alias for the file, so a send-shaped call
+          // arrives with its text in `caption`; without this alias that text is
+          // silently dropped instead of becoming the upload's first comment.
+          readStringParam(actionParams, "caption", { allowEmpty: true }) ??
           "",
         filename: readStringParam(actionParams, "filename"),
         title: readStringParam(actionParams, "title"),