feat: add async Stream API and enhance ergonomics

[isdaniel]

• Add EventStream and EventStreamRef implementing futures::Stream
• Add into_stream() and stream() methods for ergonomic event iteration
• Update ChangeEvent to have non-optional LSN field for consistency
• Add futures dependency for Stream trait support
• Enhance documentation with Stream API examples
• Improve error messages using inline format strings
• Add comprehensive unit tests for stream functionality
• Update README with dual API examples (Stream vs Polling)
• Add AsRef<[u8]> implementation for BufferWriter
• Refactor clippy warnings and improve code quality

[gemini-code-assist]

Summary of Changes

Hello @isdaniel, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the pg-walstream library by introducing a new asynchronous Stream API, leveraging Rust's futures::Stream trait. This provides a more modern and ergonomic way for users to consume PostgreSQL logical replication events, simplifying event processing and integration with other async Rust ecosystems. Alongside this major feature, the PR includes various quality-of-life improvements such as more consistent error handling, clearer documentation, and internal code refinements to ensure robustness and maintainability.

Highlights

• Asynchronous Stream API: Introduced EventStream and EventStreamRef types that implement the futures::Stream trait, providing an ergonomic, iterator-like interface for consuming replication events.
• Ergonomic Event Iteration: Added into_stream() and stream() methods to LogicalReplicationStream for easy conversion to the new Stream API, simplifying event processing with async combinators.
• ChangeEvent Consistency: The ChangeEvent struct has been updated to make the LSN (Log Sequence Number) field non-optional, ensuring every change event is consistently associated with an LSN.
• Dependency Update: The futures crate has been added as a dependency to support the new asynchronous stream functionality.
• Improved Error Messages: Error messages across various modules (buffer.rs, connection.rs, protocol.rs, types.rs) have been enhanced using inline format strings for better clarity and conciseness.
• Enhanced Documentation and Examples: The README.md and lib.rs documentation have been updated with comprehensive examples demonstrating both the new Stream API and the traditional Polling API.
• Code Quality and Testing: Refactored clippy warnings, improved internal code quality, and added comprehensive unit tests for the new stream functionality and existing components.

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[gemini-code-assist]

Code Review

This pull request primarily introduces a new Stream API for consuming PostgreSQL logical replication events, leveraging the futures::Stream trait for a more ergonomic, iterator-like interface. Key changes include adding the futures crate and related dependencies (futures-channel, futures-executor, futures-macro, futures-task, futures-util, pin-utils, slab), and updating README.md and lib.rs documentation to showcase both the new Stream API and the existing Polling API. The LogicalReplicationStream now provides into_stream() and stream() methods to create EventStream and EventStreamRef types, which implement futures::Stream and handle automatic retries, reconnections, and cancellation. Additionally, the ChangeEvent struct was modified to make the lsn field non-optional, and its constructors were updated accordingly. Minor code style improvements were made across several files, primarily updating format! macros to use the shorthand {var} syntax and refining test assertions. Review comments highlighted that the tokio::time::sleep call in the retry logic should be wrapped in a tokio::select! to ensure responsive cancellation during backoff, and that futures::Stream implementations should return Poll::Ready(None) instead of Some(Err(ReplicationError::Cancelled(...))) for graceful termination upon cancellation.

[gemini-code-assist]

The tokio::time::sleep call in the retry logic doesn't handle cancellation. If the stream is cancelled while in this sleep, it will wait for the full delay before checking the cancellation token again in the next loop iteration. This can delay graceful shutdown.

To make cancellation more responsive, you can use tokio::select! to race the sleep future against the cancellation token's cancelled() future.

                    tokio::select! {
                        biased;
                        _ = tokio::time::sleep(delay) => {},
                        _ = cancellation_token.cancelled() => {
                            return Err(ReplicationError::cancelled("Operation cancelled during retry backoff"));
                        }
                    }

[gemini-code-assist]

When the stream is cancelled, you're returning Some(Err(ReplicationError::Cancelled(...))). While this works, it's more idiomatic for a Rust stream that is gracefully terminated to return Poll::Ready(None). This signals the end of the stream to consumers like while let Some(...) loops without them needing to handle a specific error case for cancellation.

        if self.cancellation_token.is_cancelled() {
            return Poll::Ready(None);
        }

[gemini-code-assist]

When the stream is cancelled, you're returning Some(Err(ReplicationError::Cancelled(...))). While this works, it's more idiomatic for a Rust stream that is gracefully terminated to return Poll::Ready(None). This signals the end of the stream to consumers like while let Some(...) loops without them needing to handle a specific error case for cancellation. This is the same feedback as for EventStream.

        if self.cancellation_token.is_cancelled() {
            return Poll::Ready(None);
        }