test(test_fetch_json_db.py): improve test coverage for fetch_json_db

[JigyasuRajput]

Description:

Fixes #4876

This PR adds unit tests for the fetch_json_db.py module, increasing test coverage and ensuring better reliability. The following test cases have been implemented:

1. get_failed_downloads

• Ensures that missing JSON files are correctly identified when only some files are present.

2. verify_signature

• Tests signature verification using mock public keys and signature files.
• Covers cases for:
  • Valid signatures.
  • Invalid signatures.
  • Invalid public keys.
  • Missing public key scenarios (ensuring correct error handling).

3. verify_signature_missing_signature_file

• Ensures proper error handling when the signature file is missing.

4. cleanup_directory

• Tests that temporary files and directories are correctly removed while preserving the main directory.

5. handle_download

• Covers different download scenarios:
  • Unsigned Data: Ensures early exit with a signing error when ignore_signature=False.
  • Signed Data with Invalid Signature: Checks correct error handling for invalid signatures.
  • Successful Download with Retry: Validates the retry mechanism for failed downloads.
  • Max Retries Exceeded: Ensures the function respects the retry limit and performs proper cleanup.

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Additional Details:

• Mocks are used for aiohttp.ClientSession and gnupg.GPG to simulate real-world conditions.
• A MockResponse class is implemented to handle HTTP responses in test cases.

[JigyasuRajput]

fixed the flake8 and black which was failing