fix(evals): extract bucket name from eval_storage_uri when a path is given

[chelsealong]

Fixes #6881

Problem

create_gcs_eval_managers_from_uri()'s docstring says:

eval_storage_uri: The evals storage URI to use. Supported URIs: gs://<bucket name>. If a path is provided, the bucket will be extracted.

But the implementation never extracted a bucket from a path — it treated
everything after gs:// as the literal bucket name:

gcs_bucket = eval_storage_uri.split('://')[1]

gs://my-bucket/some/path produced the invalid bucket name
my-bucket/some/path (slashes included), which is then passed to
GcsEvalSetsManager/GcsEvalSetResultsManager and fails later with a
misleading "bucket does not exist" error, even when my-bucket itself
exists.

Fix

Split off the first path segment so only the bucket name is used:

gcs_bucket = eval_storage_uri.split('://')[1].split('/')[0]

This matches the docstring's documented behavior for gs://<bucket name>
URIs. As noted in the issue, threading the remaining path through as a
storage prefix is a separate, larger change (the eval history/sets
directory helpers currently hard-code their paths) and is left out of this
minimal fix.

Testing plan

Added test_create_gcs_eval_managers_from_uri_extracts_bucket_from_path to
tests/unittests/cli/utils/test_evals.py, asserting that
gs://test-bucket/some/path results in GcsEvalSetsManager and
GcsEvalSetResultsManager being constructed with bucket_name='test-bucket'.

Confirmed the new test fails against the old code:

FAILED tests/unittests/cli/utils/test_evals.py::test_create_gcs_eval_managers_from_uri_extracts_bucket_from_path - AssertionError: expected call not found.
Expected: GcsEvalSetsManager(bucket_name='test-bucket', project='test-project')
  Actual: GcsEvalSetsManager(bucket_name='test-bucket/some/path', project='test-project')

And passes with the fix, along with the full existing suite for this module:

$ pytest tests/unittests/cli/utils/ -q
421 passed, 2 xfailed, 143 warnings in 17.90s

Also verified pyink --check and isort --check are clean on both changed
files.

AI disclosure

This change was authored with the assistance of an AI coding agent (Claude),
with the diff reviewed before submission.