fix: preserve uncertainty across append retries

[devin-ai-integration]

Summary

Python analog of s2-streamstore/s2#767.

When an append attempt fails indefinitely (e.g. unavailable) and a retry then fails definitively (e.g. rate_limited), the SDK raised only the final error, which reads as "no side effect" — even though the earlier attempt may have become durable.

Unary appends (Retrier(track_append_uncertainty=True)) and append sessions (per _InflightInput.prior_uncertainty) now remember that a retried attempt may have taken effect and, when the final error would otherwise look definite, raise

class AppendIndefiniteFailureError(S2Error):
    final_attempt_error: Exception  # also set as __cause__

Already indefinite final errors and successful retries are unchanged; acknowledged inputs drop their uncertainty when they leave inflight_inputs.

Link to Devin session: https://app.devin.ai/sessions/cbd9b244893a445ea0a9a49dc0cd0fe9
Open in Devin Desktop: https://app.devin.ai/desktop/session/cbd9b244893a445ea0a9a49dc0cd0fe9?variant=devin
Requested by: @sgbalogh

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge with no outstanding findings.

Findings

1.  Unsent entries inherit uncertainty ▶

Fix with agent prompt

### Issue 1
src/s2_sdk/_s2s/_append_session.py:154-156
This marks every inflight entry uncertain when any request frame was flushed. An entry is added to `inflight_inputs` before its own frame is yielded and flushed, so batch B can be marked merely because batch A was sent. If a retry acknowledges and removes A, then ends with a definite failure before B is sent, B's stale flag causes `AppendIndefiniteFailureError` even though all possibly durable data was acknowledged and B never left the process. Track uncertainty only for entries actually written on the failed session, as the coupled Go implementation does with per-entry [`sentOnSessions`](https://github.com/s2-streamstore/s2-sdk-go/blob/HEAD/s2/append_session.go).

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Summary

This PR preserves append uncertainty across retries so a definitive final error cannot incorrectly imply that earlier attempts had no side effects.

• Adds and publicly exports AppendIndefiniteFailureError, retaining the final attempt error as both an attribute and cause.
• Tracks uncertainty in unary append retries and append sessions.
• Adds coverage for definitive and indefinite retry outcomes.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Begin append attempt] --> B{Attempt result}
    B -->|Success| C[Return success]
    B -->|Indefinite failure and retryable| D[Record prior uncertainty]
    B -->|Definite failure and retryable| E[Retry without adding uncertainty]
    D --> F[Retry append]
    E --> F
    F --> B
    B -->|Terminal indefinite failure| G[Raise original indefinite error]
    B -->|Terminal definite failure| H{Prior uncertainty?}
    H -->|No| I[Raise final error]
    H -->|Yes| J[Raise AppendIndefiniteFailureError]

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