[Feature] Add tests for streaming basis encoder coverage

[viiccwen]

ref: #1058

What

Add focused tests for qdp/qdp-core/src/encoding/basis.rs.

Current coverage for this file is still zero:

• Function coverage: 0.00% (0/6)
• Line coverage: 0.00% (0/108)

Why

The streaming basis encoder contains CPU-side validation that should be stable and easy to verify, but none of it is covered right now.

That includes:

• invalid sample size handling
• negative and non-integer basis indices
• out-of-range basis indices
• the needs_staging_copy() == false path

How

Add tests under qdp/qdp-core/tests/ that exercise the basis streaming encoder directly.

Suggested coverage targets:

• assert needs_staging_copy() returns false
• reject sample_size != 1
• reject non-finite, negative, fractional, and out-of-range basis indices
• verify buffer reuse behavior across chunks if practical
• add one successful encode path for a small valid input

This issue is done when encoding/basis.rs is no longer completely uncovered in cargo llvm-cov.

[yehfelareborn]

take,I want to contribute this issue