feat(model-selection): refine load balanced model text

What does this merge request do and why?

Follow-up to fix: add missing provider label for load-balanc... (!6634 - merged) • Julie Huang • 19.4

This change adds more specifics around how the load balanced default model works based off of the discussion in https://gitlab.com/gitlab-org/gitlab/-/work_items/613150#note_3733295874

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Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change may affect agent quality, consider triggering the manual sanity-tests job (SWE-bench eval on CEF) at your discretion. It takes ~30 min and uses significant compute/tokens, and the result is not a pass/fail signal, it requires manual analysis in LangSmith. Not needed for dev tooling, infra, observability, docs, or CI changes.
• If this change requires executor implementation: verified that issues/MRs exist for the Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.