display Record Not Found for invalid record IDs

[arvindselvajas0222-coder]

Fixes #4896

Technical details
This PR updates the Record page logic to correctly handle the case where a record ID does not exist.

Previously, when the API returned a 404 for a non-existent record, the frontend continued rendering the record form with empty values, which appeared as an editable “unsaved” record. This caused confusing behavior.

Changes included in this PR:

• Wrapped the getRecord API call in RecordPage.svelte inside a try/catch block.
• Specifically check for error.status === 404 and set a notFound state.
• When notFound is true, render the existing <NotFound /> component instead of the record form.
• Ensures consistent behavior with the Schema Not Found page.

This aligns the record page behavior with other “Not Found” pages in Mathesar.
Screenshots

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

Developer Certificate of Origin

Developer Certificate of Origin

Developer Certificate of Origin
Version 1.1

Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
1 Letterman Drive
Suite D4700
San Francisco, CA, 94129

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.


Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified
    it.

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.

[Anish9901]

Hey @arvindselvajas0222-coder, There are a lot of unrelated code changes in PR.

• Revert all the unrelated changes and make as minimal changes as possible for fixing the issue.
• Fix failing CI tests so that your PR can be reviewed.