docs: remove non-existent request_counts and expired status from async example

[drunkcoding]

Description

The polling example in docs/async-batch-submission.md referenced a batch field and a status value the server never produces:

1. b.get("request_counts", {}) — BatchObject (batchgen/server/io_struct.py:177) has no request_counts field (nor any per-request completed/total counts), so the example always printed 0/0 ok regardless of real progress. Removed the line and the count from the print.
2. "expired" status (two terminal-status tuples) — BatchStatus (io_struct.py:154) defines only validating, in_progress, completed, failed, cancelling, cancelled. "expired" never matches. Removed it from both tuples.

The example still prints status + elapsed + output_file_id, all of which are real BatchObject fields.

Motivation

Copy-pasting the example gave silently-wrong progress output. Resolves #279.

close #279

Type of Change

• model
• kernel
• core
• fix
• infra
• docs — documentation only

File changes

File	Δ	Note
docs/async-batch-submission.md	mod	Drop request_counts usage + "expired" status from the polling example

Checklist

• Read CONTRIBUTING + PR Merge Policy Contract.
• Tests — N/A, docs only.
• Documentation updated.

PR Merge Policy Contract — pre-merge checklist

• git diff --stat origin/main reviewed; every change traces to the task (§3.1).
• Exactly one Type of Change (docs); only docs/** touched (§2.5).
• File changes table matches the diff.
• No hygiene violations (§1).
• One concern only; surgical diff (§3).
• Commit clean: Angular format, no Co-Authored-By (§4).
• Fields verified against io_struct.py (BatchObject, BatchStatus).
• CI green — pending run.