Add new data test

[keenanjohnson]

Adds HTTP-level test coverage for the /data handler and the CSV/JSON formatting paths added in PR #8, plus targeted coverage for the reflect-based record-merging logic.

What changed

internal/data/data.go — two small testability seams, no behavior change:

• New recordIterator interface (Next / Record / Err); the real *api.QueryTableResult already satisfies it.
• New fetchPoints package-level function variable wrapping the DB call so Handle can be exercised end-to-end without a live InfluxDB.
• Extracted collectPoints(res recordIterator) map[string]*Data from Handle so the iteration branches are independently testable.

internal/data/data_test.go — 10 new tests:

• TestHandle_MethodNotAllowed — non-GET returns 405.
• TestHandle_BadStart_Returns400WithBody — missing and unparseable start both return 400 with a non-empty body.
• TestHandle_DefaultReturnsJSON — default response has Content-Type: application/json and decodes cleanly.
• TestHandle_FormatCSV_ReturnsCSV — ?format=csv returns Content-Type: text/csv and a valid CSV body.
• TestHandle_AcceptCSV_ReturnsCSV — Accept: text/csv produces the same.
• TestHandle_FetchError_Returns500 — error path from fetchPoints surfaces as 500.
• TestWriteCSV_HeaderRowAndColumnOrder — first CSV row equals csvHeaders exactly.
• TestCollectPoints_SkipsMissingHostTag — records without a host tag are dropped.
• TestCollectPoints_SkipsNilValue — records with Value() == nil do not panic and do not overwrite the field.
• TestCollectPoints_PopulatesKnownFields — mapped fields land in the right struct fields; unknown fields are ignored.

Notes / follow-up

The nil-Value test pins current behavior: when Value() is nil but the record has a valid host, a placeholder Data{Host: ...} entry is still created in the map (the write happens before the nil guard in Handle's loop). The test documents this rather than changing it — worth a separate decision on whether to move the placeholder write below the nil check.

[Copilot]

Pull request overview

Adds testability seams around the /data handler and expands unit/HTTP test coverage for JSON, CSV, error handling, and record-merging behavior.

Changes:

• Introduces recordIterator, fetchPoints, and collectPoints to isolate DB querying from record collection.
• Updates Handle to call the new fetchPoints seam before JSON/CSV response formatting.
• Adds handler, CSV, and collectPoints-focused tests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
internal/data/data.go	Extracts data fetching/collection logic behind testable seams while preserving response formatting flow.
internal/data/data_test.go	Adds HTTP handler tests, CSV output tests, and targeted collectPoints tests using fake Flux records.

Comments suppressed due to low confidence (1)

internal/data/data_test.go:223

• This nil-value test only checks the zero-value field on a newly created placeholder, so it would not catch a regression that overwrites an existing nonzero CO2 value with zero when a later nil record for the same host/time arrives. Seed the iterator with a valid value before the nil record to verify the "does not overwrite" behavior described by the assertion.

	require.Len(t, points, 1, "a placeholder Data entry is created before the nil-value check")
	for _, d := range points {
		require.Equal(t, "frog-01", d.Host)
		require.Equal(t, 0.0, d.CO2, "nil Value() must not be written into the field")

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