test: Add tests for QueryEngine and utils for query package.

[Ta5r]

What

• This PR adds jest tests for testing of the QueryEngine present in the snapwp/query package.
• This PR also tests the parse-template and parse-global-styles utils present.

Why

• This PR is necessary to fully validate the correct setup of QueryEngine, particularly ensuring that it initializes only once and correctly configures the Apollo Client.
• The parseQueryResult functions for templates and global styles needed test cases to verify correct data parsing and error handling.

Related Issue(s):

• Part of https://github.com/rtCamp/headless/issues/279

How

• Adds assertions to verify QueryEngine initializes correctly and remains a singleton.
• Adds tests for parseQueryResult functions by:
  • Validating successful parsing of template and global styles data.
  • Ensuring errors are logged and thrown correctly when data is missing or invalid.

Testing Instructions

• Run :

npm run test:unit

Screenshots

Additional Info

NA

Checklist

• I have read the Contribution Guidelines.
• My code is tested to the best of my abilities.
• My code passes all lints (ESLint, tsc, prettier etc.).
• My code has detailed inline documentation.
• I have added unit tests to verify the code works as intended.
• I have updated the project documentation accordingly.