fix(interactive): resolve --label-column case-insensitively before pre-fill

[LukasWodka]

Summary

A supplied --label-column that matches a CSV header only by case silently
selected the wrong column.

defaultInOptions matches exactly (o == want), so --label-column Income
against a header income failed the match and fell through to
defaultLabelChoice, which returns the first column when nothing is named
label. The Select then opened on that wrong column and Enter accepted it.

Until #505 a supplied --label-column skipped the prompt entirely and its
spelling was kept verbatim, so it never had to agree with the header. Making the
question always-ask is what made the mismatch reachable.

canonicalHeader resolves the supplied value to the header's own spelling before
the guard runs. Resolving there rather than loosening defaultInOptions is
deliberate: that helper also guards --intent and --label-policy against fixed
vocabularies, which should stay exact matches. A value matching nothing is
returned unchanged, so a genuine typo still falls back rather than being
case-folded into a hit.

Type

fix

Test plan

Two rows added to TestPromptLabelColumn_SuppliedDefaultPrefillsAndGuards
(fixture header age,income,churned, no column named label):

• Income → income
• CHURNED → churned

Mutation-proved. Reverting the canonicalHeader call reddens both, and with
exactly the defect's signature:

promptLabelColumn(supplied="Income")  = "age", want "income"
promptLabelColumn(supplied="CHURNED") = "age", want "churned"

go test ./... green across all 18 packages; go vet and gofmt clean.

Provenance

Found by Bugbot on release-train promotion PR #511 (Medium). Per the staging-hop
policy in release-train's README, findings at develop -> staging are fixed on
the source branch rather than ticketed. This unblocks the cli leg of the current
staging hop.

Checklist

• Targets develop
• Single self-contained change
• Tests added and mutation-proved
• No secrets or customer data

---

Note

Low Risk
Narrow interactive CLI pre-fill fix with targeted tests; no changes to ingest validation or server-side label handling.

Overview
Fixes a regression where --label-column differing from the CSV header only by case pre-selected the wrong column in the guided label picker.

After #505 always asks the label question, defaultInOptions requires an exact header match, so values like Income against income missed and fell through to defaultLabelChoice (first column when no label column exists). canonicalHeader maps the supplied flag to the header’s spelling via strings.EqualFold before that guard; real typos still miss and use the same fallback. defaultInOptions stays strict for fixed vocabularies (--intent, --label-policy).

Tests in TestPromptLabelColumn_SuppliedDefaultPrefillsAndGuards cover Income → income and CHURNED → churned.

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