test: add unit tests for IDE DMA engine

[Copilot]

This PR adds comprehensive unit tests for the IDE DMA engine to improve test coverage of the complex DMA functionality that was modified in #633.

Changes

Added 5 focused unit tests that exercise the core DMA engine functionality in perform_dma_memory_phase:

1. test_dma_read_multiple_prds - Tests DMA read operations with multiple Physical Region Descriptors (PRDs), ensuring the DMA engine correctly processes descriptor chains
2. test_dma_write_multiple_prds - Tests DMA write operations with multiple PRDs, validating data transfer across multiple memory regions
3. test_dma_read_zero_byte_count - Tests the edge case where a PRD has zero byte count, which should be interpreted as 64KB according to the IDE specification
4. test_dma_descriptor_read_error - Tests error handling when DMA descriptor table reads fail due to invalid memory addresses
5. test_dma_prd_boundary_handling - Tests PRD boundary logic and descriptor chaining with different sized regions

Background

The IDE DMA engine is a complex component that handles Physical Region Descriptor (PRD) tables for scatter-gather DMA operations. Issue #633 fixed a bug related to PRD exhaustion during write operations, but this area lacked comprehensive unit tests, making it difficult to reason about future changes.

These tests use the existing enlightened interface to trigger DMA operations while focusing on different DMA scenarios and edge cases that exercise the core engine logic.

Testing

All new tests pass and existing IDE tests continue to work:

running 14 tests
test drive::hard_drive::tests::test_lba ... ok
test tests::identify_test_cd ... ok
test tests::enlightened_cmd_test_incomplete_prd ... ok
test tests::enlightened_hdd_cmd_test ... ok
test tests::enlightened_cd_cmd_test ... ok
test tests::identify_test_hdd ... ok
test tests::software_reset_test ... ok
test tests::read_sectors_test ... ok
test tests::test_dma_descriptor_read_error ... ok
test tests::test_dma_prd_boundary_handling ... ok
test tests::test_dma_read_zero_byte_count ... ok
test tests::test_dma_read_multiple_prds ... ok
test tests::test_dma_write_multiple_prds ... ok
test tests::write_sectors_test ... ok

test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Fixes #1587.

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