fix: append session should also create-stream-on-append if configured

[shikhar]

No description provided.

[greptile-apps]

Greptile Overview

Greptile Summary

Fixed append_session to honor the create_stream_on_append basin configuration, bringing it into parity with the regular append method. Previously, append_session called streamer_client() directly, which would fail with StreamNotFoundError when the stream didn't exist, even if the basin was configured to auto-create streams on append.

• Changed lite/src/backend/append.rs:42-46 to use streamer_client_with_auto_create instead of streamer_client
• Added comprehensive test test_append_session_auto_create_stream to verify the fix
• Test confirms stream is created automatically and append succeeds when create_stream_on_append = true

This ensures consistent behavior between both append methods and matches user expectations for the auto-create feature.

Confidence Score: 5/5

• This PR is safe to merge with no risk
• The change is a straightforward bug fix that brings append_session into parity with append. The implementation correctly reuses the existing streamer_client_with_auto_create method with the same configuration check. The new test thoroughly validates the fix and follows existing test patterns.
• No files require special attention

Important Files Changed

Filename	Overview
lite/src/backend/append.rs	Updated append_session to respect create_stream_on_append configuration, matching the behavior of the regular append method
lite/tests/backend/data_plane/append.rs	Added test test_append_session_auto_create_stream to verify that append_session auto-creates streams when configured

Sequence Diagram

sequenceDiagram
    participant Client
    participant Backend
    participant StreamerClient
    participant Basin

    Client->>Backend: append_session(basin, stream, inputs)
    Backend->>Backend: streamer_client_with_auto_create()
    Backend->>Basin: get_basin_config()
    Basin-->>Backend: BasinConfig (create_stream_on_append)
    
    alt Stream exists
        Backend->>StreamerClient: get existing client
        StreamerClient-->>Backend: client
    else Stream not found AND create_stream_on_append = true
        Backend->>Backend: create_stream()
        Backend->>StreamerClient: get new client
        StreamerClient-->>Backend: client
    else Stream not found AND create_stream_on_append = false
        Backend-->>Client: StreamNotFoundError
    end
    
    Backend->>Backend: SessionHandle::new()
    Backend-->>Client: Stream<Result<AppendAck, AppendError>>
    
    loop for each input
        Client->>StreamerClient: append_permit(input)
        StreamerClient->>StreamerClient: submit_session()
        StreamerClient-->>Client: AppendAck
    end

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