test: strengthen fixture-backed stream coverage

[rmorse]

Summary

• Add crate-local test fixture resolution for source-level unit tests so they find the vendored whisper.cpp/models fixtures populated by cargo xtask test-setup.
• Add a real-audio WhisperStream::flush integration test using samples/jfk.wav and the tiny.en model.
• Correct the type-safety test to assert the actual FullParams/TranscriptionParams Send+Sync contract.
• Tighten stream config normalization coverage so it exercises the real constructor when fixtures are available.

Coverage impact

• Wrapper crate coverage moved from 71.01% lines / 70.92% functions / 67.65% regions to 80.59% lines / 77.57% functions / 80.61% regions.
• whisper-cpp-plus/src/stream.rs moved from 37.67% line coverage to 85.87% line coverage.
• whisper-cpp-plus/src/stream_pcm.rs moved from 86.50% line coverage to 89.30% line coverage.

Validation

• cargo test -p whisper-cpp-plus --test stream_integration -- --test-threads=1
• cargo test -p whisper-cpp-plus --lib -- --test-threads=1
• cargo test -p whisper-cpp-plus --test type_safety -- --test-threads=1
• cargo test -p whisper-cpp-plus --features async --lib -- --test-threads=1
• cargo clippy -p whisper-cpp-plus --all-targets -- -D warnings
• cargo clippy -p whisper-cpp-plus --all-targets --features async -- -D warnings
• cargo test -p whisper-cpp-plus -- --test-threads=1
• cargo test -p whisper-cpp-plus --features async -- --test-threads=1
• cargo llvm-cov -p whisper-cpp-plus --json --summary-only --output-path D:\cargo-coverage-whisper-cpp-plus-rs\coverage-whisper-cpp-plus-stream-fixtures.json -- --test-threads=1
• cargo fmt --all -- --check
• git diff --check