[Feature] Add tests for streaming amplitude encoder coverage

[viiccwen]

ref: #1058

What

Add focused tests for qdp/qdp-core/src/encoding/amplitude.rs so the streaming amplitude encoder is covered by cargo llvm-cov.

Current coverage for this file is still effectively zero:

• Function coverage: 0.00% (0/4)
• Line coverage: 0.00% (0/73)

Why

encoding/amplitude.rs contains the streaming Parquet encoding path, but none of its validation or chunk-level behavior is currently exercised in the Rust coverage run.

That means we do not have regression protection for:

• sample size validation
• state initialization
• chunk encoding behavior
• kernel launch error propagation

How

Add tests under qdp/qdp-core/tests/ that exercise AmplitudeEncoder through the streaming path used by crate::encoding::stream_encode.

Suggested coverage targets:

• reject sample_size == 0
• reject sample_size > STAGE_SIZE_ELEMENTS
• successful init_state() allocation for a valid chunk size
• at least one end-to-end streaming encode case that reaches encode_chunk()
• if practical, one failure case that asserts MahoutError::KernelLaunch is surfaced correctly

This issue is done when qdp/qdp-core/src/encoding/amplitude.rs is no longer at 0% coverage in cargo llvm-cov.

[0lai0]

take