fix(protocols): accept null stream option flags

[furionw]

Summary

• Treat missing or explicit-null include_usage and continuous_usage_stats as false.
• Preserve the public bool fields and their serialized boolean wire shape.
• Continue rejecting non-boolean, non-null values.

This moves the compatibility behavior into the shared protocol type used by chat and text completion requests. It is the durable protocol-side fix for the stream-options portion of ai-dynamo/dynamo#14847; after Dynamo consumes a release containing this change, its frontend-only nullable stream-options wrapper can be removed.

Compatibility

This is a deserialization-only relaxation. Existing Rust constructors remain source-compatible, JSON and MessagePack serialization remain boolean-valued, and mixed-version request-plane traffic retains the existing wire shape.

Validation

• cargo fmt --all -- --check
• cargo test -p dynamo-protocols --all-targets --locked
• cargo test -p dynamo-protocols --doc --locked
• cargo clippy -p dynamo-protocols --all-targets --all-features --locked -- -D warnings
• cargo check --workspace --all-targets --locked
• cargo test --workspace --all-targets --locked --exclude dynamo-conformance-fixtures-v2
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• Directed MessagePack compatibility check for old boolean payloads, unchanged output shape, and null normalization

The unfiltered workspace test run reached the conformance package and stopped because the local macOS environment does not provide flock; the same workspace run excluding that environment-specific fixture package passed.