fix(tools/looker): address CI failures and enhance agent tool functionality

[hiracky16]

…ort agent instructions/update

Description

This PR addresses CI failures observed in #2830 and enhances the looker-agent tool with missing functionality and improved robustness.

Based on the CI results in #2830, I have fixed the linting issues and the documentation build error. I have also taken this opportunity to implement essential missing features such as agent instructions and update operations.

PR Checklist

Thank you for opening a Pull Request! Before submitting your PR, there are a
few things you can do to make sure it goes smoothly:

• Make sure you reviewed
  CONTRIBUTING.md
• Make sure to open an issue as a
  bug/issue
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)
• Make sure to add ! if this involve a breaking change

🛠️ Fixes #<issue_number_goes_here>

[gemini-code-assist]

Code Review

This pull request simplifies the documentation for the Looker agent's sources parameter and refactors lookeragent_test.go by shortening error message expectations, reformatting mock methods, and removing trailing whitespace. Feedback was provided to enhance the documentation by including specific details about the expected JSON keys ('model' and 'explore') for data sources, as the current change reduced the informativeness of the description.

[drstrangelooker]

/gcbrun

[github-actions]

🔎 Preview at https://googleapis.github.io/genai-toolbox/previews/PR-2887/