fix(gateway): harden message action channel fallback and startup grace

Take the safe, tested subset from #32367:\n- per-channel startup connect grace in health monitor\n- tool-context channel-provider fallback for message actions\n\nCo-authored-by: Munem Hashmi <munem.hashmi@gmail.com>

src/infra/outbound/message-action-runner.test.ts

@@ -349,6 +349,37 @@ describe("runMessageAction context isolation", () => {
     expect(result.channel).toBe("slack");
   });
 
+  it("falls back to tool-context provider when channel param is an id", async () => {
+    const result = await runDrySend({
+      cfg: slackConfig,
+      actionParams: {
+        channel: "C12345678",
+        target: "#C12345678",
+        message: "hi",
+      },
+      toolContext: { currentChannelId: "C12345678", currentChannelProvider: "slack" },
+    });
+
+    expect(result.kind).toBe("send");
+    expect(result.channel).toBe("slack");
+  });
+
+  it("falls back to tool-context provider for broadcast channel ids", async () => {
+    const result = await runDryAction({
+      cfg: slackConfig,
+      action: "broadcast",
+      actionParams: {
+        targets: ["channel:C12345678"],
+        channel: "C12345678",
+        message: "hi",
+      },
+      toolContext: { currentChannelProvider: "slack" },
+    });
+
+    expect(result.kind).toBe("broadcast");
+    expect(result.channel).toBe("slack");
+  });
+
   it("blocks cross-provider sends by default", async () => {
     await expect(
       runDrySend({