Handle no grounding responses better ai 1697

[dearden]

Fixes AI-1697.

Currently we ask Gemini to do grounding a grounding Google search. If it doesn't do that search, we will still return the output.

This change makes it so that if Gemini doesn't do that search, we raise an exception.

Practically, this will mean that SIFT should stop showing results with made up URLs. It will also mean that for an unknown number of claims we may not get any results, but we've decided that's probably better than potentially having lots of hallucinated sources.

There's also some improvements to test coverage, at the end of test_gemini.py. They're fairly uncontroversial, but there were various things I wanted to check to make sure this new exception didn't change behaviour in unexpected ways.

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Pull request checklist

• I have linked my PR to an issue
• I’ve used conventional commits
• My branch is up-to-date with main
• Where appropriate, I have added or updated tests
• Where appropriate, I have updated documentation to reflect my changes

[nmenezes0]

Looks good - nice clear function names so I think I managed to follow!