test: RFMTransformer.fit never exercises the ndarray (non-Da

[Mr-Neutr0n]

Fixes #49

Added test_rfm_transformer_ndarray_input_raises_missing_columns to cover RFMTransformer.fit’s bare-ndarray path and verify it raises ValueError for missing required columns.

Local tests pass.

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This change was prepared with AI assistance under human direction and review.

[shivamlalakiya]

Test looks correct and covers the missing-columns ndarray path cleanly. Per AGENTS.md, every PR needs a CHANGELOG.md entry under ## [Unreleased], and since this lands your first contribution, a one-line add-yourself in CONTRIBUTORS.md too (see the 'Getting listed' section there). Could you add both? Holding the merge until then.

[shivamlalakiya]

The test itself is good: six lines, closes the real gap in issue #49, and it pins the donor_id message rather than a bare ValueError, so a regression that swaps the error text still fails the test.

Blocking on one repo rule, not on the code: AGENTS.md requires every PR to add its author to CONTRIBUTORS.md in the same PR, and @Mr-Neutr0n is not listed there yet. Please add yourself (see the "Getting listed" section) and I will merge.

No CHANGELOG entry needed here -- test-only PRs (#107, #118) have merged without one, and PR #128 is adding a CI guard that scopes the changelog requirement to changes under philanthropy/.

[Mr-Neutr0n]

Added myself to CONTRIBUTORS.md per the AGENTS.md rule: fc14ccc. Ready for merge.