fix(utils): embed(texts=[]) crashes with IndexError on empty input

[devteamaegis]

What's broken

Client.embed(texts=[]) (default batching, no images) produces an empty responses list, which merge_embed_responses indexes unconditionally:

>>> import cohere
>>> co = cohere.Client(api_key="fake-no-network")
>>> co.embed(texts=[])
IndexError: list index out of range   # src/cohere/utils.py, responses[0].response_type

The crash happens before any network I/O, so an empty input list never gets a meaningful error.

Why it happens

merge_embed_responses reads responses[0] without checking for an empty list.

Fix

Guard the empty case and raise a clear ValueError. The SDK can't synthesise a typed empty EmbedResponse (it wouldn't know response_type), so an explicit, actionable error is the smallest correct behavior and matches the library's existing input-validation pattern.

Test

Added a case asserting merge_embed_responses([]) raises ValueError instead of IndexError.

Fixes #777

---

Note

Low Risk
Localized input-validation change in embed merge utilities with a regression test; version bump only affects release metadata and User-Agent strings.

Overview
Fixes Client.embed(texts=[]) crashing with IndexError when batched embed yields no responses and merge_embed_responses accessed responses[0]. An early guard now raises a clear ValueError instead of failing opaquely (before any network call).

Adds a unit test for merge_embed_responses([]) and bumps the package/SDK version to 7.0.4 (including Fern metadata and HTTP client version headers).

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