Trim comments per review on #897

[devin-ai-integration]

Summary

Comment-only follow-up: #897 merged before the review nits landed, so the two suggested trims are applied here verbatim — resolveProviderFor's docstring collapses to naming the condition (TierUnknown), and the TestCandidateK12Loads note keeps only the non-obvious why (the win mix is unchanged). No behavior change.

Link to Devin session: https://app.devin.ai/sessions/f35da828b04946a7a6e2b0d986b6ce45
Requested by: @steventohme

[devin-ai-integration]

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[claude]

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