Fix tests

[markmcd]

Fix tests across all languages

• Use GEMINI_API_KEY instead of GOOGLE_API_KEY everywhere (except Java, which has it mapped)
• JDK 21 -> 24
• Python unittest framework
• Increase Go test timeout so it runs

Also removing the "environment" setup, it's too much overhead. Using maintainer-approval button to kick off tests instead.

There are some failing tests that should be corrected in #101.

[gemini-code-assist]

Code Review

This pull request updates the model ID to gemini-2.5-flash and removes the explicit environment variable check for GEMINI_API_KEY in javascript/grounding.js. Feedback indicates that removing this validation reduces the robustness of the example and could lead to inconsistent behavior or confusing error messages if the required environment variable is missing.

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javascript/grounding.js (21-23)

Removing the explicit environment variable check reduces the robustness of the example and potentially contradicts the stated goal of standardizing on GEMINI_API_KEY. Without this validation, if GEMINI_API_KEY is missing, the SDK may fall back to looking for GOOGLE_API_KEY in the environment or attempt to use Application Default Credentials, leading to inconsistent behavior and confusing error messages for users. Maintaining a clear 'API key not set' error is a best practice for developer experience in example code.

[markmcd]

Ignore the failed environmental errors in this PR, I'm shutting that down. Actual tests are tracked in: https://github.com/google-gemini/api-examples/actions/runs/25146625460 and some have genuine failures that are being fixed in #101.