fix: improve error when extracted completion is not a list

[AdithyaKotian]

What this PR does

Improves the error message raised when a non-list completion is extracted from a trace.

Why

Currently, when a trace contains a single completion object instead of a list, the error message does not provide enough context to understand the root cause. This change makes the failure easier to debug by:

• Reporting the actual type
• Showing a truncated representation of the value
• Clearly explaining the likely cause and expected format

Scope

• No behavior change
• No automatic normalization
• Error message only

Fixes #404

[ultmaster]

Revert this as it's not related to this PR.

[ultmaster]

/ci

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