[CHORE] Investigate and update deprecated Cohere models in CI tests

[tazarov]

Context

Test EF (cohere) is failing CI as of 2026-04-13 due to upstream Cohere model changes. The failure is environmental, not a regression — it affects both main and any open PR.

Surfaced during PR #506 (phase 25 follow-up). The PR was merged with this single failing check after confirming it's identical on main (same code path, same secrets).

Failures observed

1. embed-english-v2.0 removed by Cohere on 2026-04-04

Used as DefaultEmbedModel (pkg/embeddings/cohere/cohere.go:42).

Cohere's own error message:

model 'embed-english-v2.0' was removed on April 4, 2026. See https://docs.cohere.com/docs/deprecations#2026-04-04-embed-v20-aya-expanse-8b for a list of models you can use instead.

This breaks Test_ef/Test_Create_Embed (which exercises the default model).

2. embed-multilingual-v3.0 returns 404 from Cohere

Hardcoded in pkg/embeddings/cohere/cohere_test.go at lines 42, 55, 72, 90, 105.

Cohere returns:

model 'embed-multilingual-v3.0' not found, make sure the correct model ID was used and that you have access to the model.

This may be:

• A subscription-tier access issue (CI's COHERE_API_KEY doesn't have v3.0 access)
• A separate deprecation that lacks a public notice
• A regional / org-scoped enablement issue

Needs investigation against Cohere's current docs and the CI key's tier.

Suggested actions

1. Pick a non-deprecated default model from https://docs.cohere.com/docs/cohere-embed (likely embed-v4.0 or current embed-multilingual-v3.0 if access can be enabled).
2. Update DefaultEmbedModel in pkg/embeddings/cohere/cohere.go.
3. Update the 5 hardcoded embed-multilingual-v3.0 references in pkg/embeddings/cohere/cohere_test.go.
4. Verify the chosen model(s) work against the CI's COHERE_API_KEY; if not, rotate or upgrade the key.
5. Update model constant docs in pkg/embeddings/cohere/option.go (lines 46, 49, 60, 63 reference v2.0/v3.0).
6. Add a short comment in cohere.go linking to Cohere's deprecation page so future maintainers know where to check.

Failing job

https://github.com/amikos-tech/chroma-go/actions/runs/24361858259/job/71143456290

Affected files (read-only list)

• pkg/embeddings/cohere/cohere.go (DefaultEmbedModel constant)
• pkg/embeddings/cohere/cohere_test.go (5 hardcoded model strings)
• pkg/embeddings/cohere/option.go (doc comments)