feat: add flush method to Producer

[quettabit]

No description provided.

[quettabit]

@greptileai review pls

[greptile-apps]

Greptile Summary

Adds a public Producer.flush() method that submits the current partial batch and waits until all previously submitted records are acknowledged.

• Tracks acknowledgement completion with one future per unacknowledged batch.
• Renames internal flush-related state and methods to distinguish batch submission from acknowledgement draining.
• Adds coverage for flushing full and partial batches while continuing to use the producer afterward.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
src/s2_sdk/_producer.py	Adds flush semantics and per-batch acknowledgement-completion tracking while preserving terminal-error propagation.
tests/test_stream_ops.py	Verifies that flush submits a partial batch, resolves earlier tickets, and permits subsequent submissions.

Sequence Diagram

sequenceDiagram
    participant Caller
    participant Producer
    participant Session
    participant Drainer
    Caller->>Producer: flush()
    Producer->>Session: submit accumulated batch
    Session-->>Producer: BatchSubmitTicket
    Producer->>Drainer: wake acknowledgement task
    Drainer->>Session: await ticket
    Session-->>Drainer: append acknowledgement
    Drainer->>Producer: resolve batch completion future
    Producer-->>Caller: flush completes

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[quettabit]

@greptileai addressed your comments. pls review again.

[quettabit]

@greptileai what other concerns do you have? can you pls convey em through comments on relevant blocks/lines?