test(render): add unit tests for NotebookHandlerImpl

[sabhi128]

Summary of Changes

This PR expands backend test coverage by adding unit tests for NotebookHandlerImpl under internal/handlers/render/.

Changes:

• New File: internal/handlers/render/notebooks_test.go
  • Added route rendering coverage for:
    1. List route (notebooks_index template rendering).
    2. Show route (parameter mapping of notebook_id, path, and custom layout configuration options to notebooks_show template).
    3. New route (checks context layout and license dictionary injection matching DefaultLicensesJSON).

All rendering calls are tested in isolation using Mockery's testifying mocks, and redundant response code assertions have been removed.

[Copilot]

Pull request overview

This PR adds backend unit test coverage for NotebookHandlerImpl in internal/handlers/render/, validating that key notebook routes invoke the expected templates with the expected rendering data.

Changes:

• Added a new test suite covering List, Show, and New behaviors for NotebookHandlerImpl.
• Validated template name selection (notebooks_index, notebooks_show, notebooks_new) and critical data mapping (e.g., notebookId, trimmed path, isSmallFooter, and licenses).

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