Filter errors that start with Couldn't add (#42764)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
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**Related issue:** Resolves #42572

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [ ] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

## Testing

- [x] Added/updated automated tests
- [ ] Where appropriate, [automated tests simulate multiple hosts and
test for host
isolation](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/reference/patterns-backend.md#unit-testing)
(updates to one hosts's records do not affect another)

- [x] QA'd all new/changed functionality manually

changes/42572-fix-duplicate-text

@@ -0,0 +1 @@
+- Fixed duplicate text in error message when script validation fails when adding a custom package

frontend/pages/SoftwarePage/SoftwareAddPage/SoftwareCustomPackage/helpers.tests.tsx

@@ -0,0 +1,53 @@
+import { getErrorMessage } from "./helpers";
+
+jest.mock("axios", () => {
+  const actual = jest.requireActual("axios");
+  return {
+    ...actual,
+    isAxiosError: () => true,
+  };
+});
+
+describe("getErrorMessage", () => {
+  it("returns message for pending install/uninstall error", () => {
+    const err = {
+      response: {
+        status: 400,
+        data: {
+          errors: [
+            {
+              name: "Error",
+              reason:
+                "Couldn't install. Host already has a pending install/uninstall for this installer.",
+            },
+          ],
+        },
+      },
+    };
+
+    expect(getErrorMessage(err)).toBe(
+      "Couldn't add. Couldn't install. Host already has a pending install/uninstall for this installer."
+    );
+  });
+
+  it("returns message for script validation error", () => {
+    const err = {
+      response: {
+        status: 400,
+        data: {
+          errors: [
+            {
+              name: "Error",
+              reason:
+                "Couldn't add. Script validation failed: Script is too large. It's limited to 500,000 characters (approximately 10,000 lines).",
+            },
+          ],
+        },
+      },
+    };
+
+    expect(getErrorMessage(err)).toBe(
+      "Couldn't add. Script validation failed: Script is too large. It's limited to 500,000 characters (approximately 10,000 lines)."
+    );
+  });
+});

frontend/pages/SoftwarePage/SoftwareAddPage/SoftwareCustomPackage/helpers.tsx

@@ -22,7 +22,6 @@ export const getErrorMessage = (err: unknown) => {
     isAxiosError(err) &&
     (err.response?.status === 504 || err.response?.status === 408);
   const reason = getErrorReason(err);
-
   if (isTimeout) {
     return REQUEST_TIMEOUT_ERROR_MESSAGE;
   }
@@ -68,6 +67,10 @@ export const getErrorMessage = (err: unknown) => {
     );
   }
 
+  if (reason.startsWith("Couldn't add.")) {
+    return `${ensurePeriod(reason)}`;
+  }
+
   if (reason) {
     return `${ADD_SOFTWARE_ERROR_PREFIX} ${ensurePeriod(reason)}`;
   }