fix(sdk): surface proxy request validation errors

[RuchirSingh1000]

Description

createOpenAIClient currently builds useful validation errors when a request references a model that is missing from the assistant config, or when that model has no apiKeyLocation / envSecretLocations. Those errors are inside the same broad try/catch used for JSON parsing, so they are swallowed and the SDK silently forwards the original request body without continueProperties.

This separates JSON parsing fallback from assistant-model validation:

• non-JSON request bodies still pass through unchanged
• missing assistant models now fail fast with the existing error message
• models without a secret location now fail fast instead of sending an unusable proxy request

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

N/A - SDK request validation fix.

Tests

• npm.cmd test -- createOpenAIClient.test.ts --runInBand from packages/continue-sdk/typescript
• git diff --check

I also tried npm.cmd test -- --runInBand and npx.cmd tsc --noEmit from packages/continue-sdk/typescript. The focused regression test passes, but the full package checks are currently blocked in this local checkout by existing generated/local package resolution issues (../api/dist/index.js and @continuedev/config-yaml).

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Summary by cubic

Surface proxy request validation errors in the SDK by separating JSON parsing from model validation in createOpenAIClient. Non-JSON bodies pass through unchanged; missing models or models without secret locations now fail fast with clear errors.

• Bug Fixes
  • Introduced addContinuePropertiesToBody to attach continueProperties only for JSON bodies.
  • Preserve original body for non-JSON POST requests.
  • Throw clear errors for missing assistant models or missing apiKeyLocation/envSecretLocations.
  • Added unit tests covering pass-through, success, and error cases.

^{Written for commit 9739005. Summary will update on new commits.}

[github-actions]

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[RuchirSingh1000]

I have read the CLA Document and I hereby sign the CLA

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}