Fixes #4774: Add error handling for system prompt

[mdelder]

• Ensure that Claude models on VertexAI send correct prompts, even if the systemMessage is undefined.

Signed-off-by: "Michael Elder" @mdelder

Description

Provide better error handling for Gemini models that have ChatMessages[] that TextMessagePart(s) instead of just strings.

Checklist

• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created
• I don't often test, but when I do, I do it in production. Jokes aside, I have verified this locally but haven't run or added any additional unit tests for this change.

Screenshots

[ For visual changes, include screenshots. ]

Testing instructions

See #4714 to reproduce the error fixed by this Pull Request.

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[Patrick-Erichsen]

Thanks for both documenting this issue, and for the PR to resolve it! 👌

It feels like a bit of a code-smell that we have this removeSystemMessage fn in the first place, but this PR isn't the place to resolve that.

[Patrick-Erichsen]

I just pushed up a commit to resolve a small type error: https://github.com/continuedev/continue/actions/runs/14114019429/job/39744113210?pr=4863#step:8:6

I pulled down this branch and wasn't getting any error in Gemini.ts so not sure what the issue there was, but my commit resolved it, along with some default prettier formatting.