Remove single-use `joi` dependency from map-search-response in favor of a TypeScript-native approach

[!NOTE]
This issue was investigated and written by Claude Code, at the request of a maintainer.

Summary

joi is a runtime schema-validation dependency that is used in exactly one place in the codebase: src/course-home/courseware-search/map-search-response.js, to validate the courseware-search endpoint response before mapping it. It is the repository's only runtime validator (no zod/yup/superstruct/ajv), so it exists solely to support this one function. This issue proposes removing the dependency in favor of a TypeScript-native approach.

Findings

joi is a single-use dependency

• Declared in package.json ("joi": "^17.11.0").
• Imported once, at src/course-home/courseware-search/map-search-response.js:1 (const Joi = require('joi');).
• Consumer chain (already React Query based):
  useCoursewareSearchResults (src/course-home/courseware-search/data/apiHooks.ts) → mapSearchResponse(data, keyword) → endpointSchema.validate(response).

The schema validates far less than it appears to

endpointSchema declares each results[] item with top-level fields id / contentType / location / url / content, but the real payload nests all of that under result.data.*. In the fixture (test-data/mocked-response.json) a result item's keys are _index, _type, _id, data, score — so the fields the mapper actually reads (result.data.content.displayName, result.data.contentType, result.score, …) live under the data key, not at the item's top level.

Combined with:

• every item field being optional (no .required()), and
• .unknown(true) on the item object,

…the item-level schema never actually validates the content shape — those fields simply pass as unknown keys. The effective runtime guarantees are only:

• took is a required number,
• total is a required number,
• maxScore is a number or null,
• results is an array of objects.

The single "wrong format" test (mapSearchResponse({ foo: 'bar' }) throws) passes purely because required took/total are missing — none of the content-shape rules are exercised.

(Casing is not the issue: searchCourseContentFromAPI camelCases via camelCaseObject, so the top-level keys line up. The mismatch is the nesting level.)

Options

Three approaches seem reasonable; this issue intentionally does not pick one.

Option A — hand-written type guard (no new dependency)

Convert map-search-response.js to TypeScript, define SearchResponse interfaces, and add a small guard that checks took/total are numbers and results is an array, throwing NonRetryableError otherwise, returning a typed value. Removes joi and adds nothing. Matches the small amount of validation that is actually effective today.

Option B — swap joi → zod

Replace the schema with a TS-native validator whose types are inferred from the schema. Trades one runtime dependency for another; most compelling if the intent is genuinely thorough runtime validation (which would also mean fixing the nesting-level mismatch so item content is really validated).

Option C — type and cast, no runtime validation

Define the response interface and cast, dropping runtime validation entirely. Smallest change; removes joi; loses the throw-on-malformed-response guarantee (a downstream error would surface via React Query instead). One test would need to change.

Decisions to resolve

1. Which option (A / B / C).
2. Preserve the current loose behavior, or fix the dead item-level validation? A faithful port keeps validation to took/total/results; a "fix it properly" port validates result.data.* for real. These are different-sized changes.

Scope notes

• The file currently uses CommonJS require(); converting it to TypeScript aligns it with the surrounding code.
• Existing tests to keep green: src/course-home/courseware-search/map-search-response.test.js and the mapSearchResponse mock usage in src/course-home/courseware-search/data/apiHooks.test.tsx.

Start with package.json and src/course-home/courseware-search/map-search-response.js, then read map-search-response.test.js and the mapSearchResponse usage in data/apiHooks.test.tsx. Resolve with a maintainer whether to use option A, B, or C and whether to preserve or fix item-level validation. Done means joi is removed and the named tests remain green.