Add multi-stream produce capability to IProducer with batch efficiency and Gateway integration

[qodo-free-for-open-source-projects]

Description

• Current Gateway implementation groups messages by stream then produces per-stream sequentially, creating overhead for multi-stream scenarios
• Need efficient batch API for producing to multiple streams in parallel with single method call
• Generic type parameter variance constraints prevent placing multi-stream overload on contravariant IProducer<in T> interface
• Existing single-stream produce methods must remain unchanged for backward compatibility

Deliverables

• Request Types: Create ProduceRequest and ProduceRequest<TProduceOptions> record structs as input types for multi-stream operations
• Interface Methods: Add default interface methods to IProducer and IProducer<TProduceOptions> with parallel execution via Task.WhenAll
• BaseProducer Implementation: Override multi-stream methods in BaseProducer<T> to support all concrete producer implementations
• Gateway Refactor: Update GatewayHandler to batch messages by stream and use single multi-stream produce call instead of per-stream loops
• Producer Delegation: Add multi-stream delegation methods to GatewayProducer with readiness checks
• Test Coverage: Add unit tests validating multi-stream, empty requests, same-stream batching, and untyped request variants

[linear]

EVE-222