fix(release): bump crates.io verify polling 12→30 attempts (3min→7.5min)

[saurabhjain1592]

Summary

The `Verify version is live on crates.io` step in release.yml false-failed during the v0.2.0 release (workflow run 25604826136). `cargo publish` succeeded; the version was created in the crates.io DB at 15:34:21Z (verified via `created_at` field on `/api/v1/crates/axonflow-sdk-rust/0.2.0` checked manually 30 seconds after the polling window closed), but the public API endpoint served a cached 404 for the entire 3-minute polling window (15:34:23Z → 15:37:24Z).

Change

Bump the polling loop from `12 × 15s = 3min` to `30 × 15s = 7.5min`. Polling-instead-of-sleeping preserves the fast-exit on typical fast-propagation cases (30-60s).

DoD (dryrun)

Ran the new polling logic locally against the live v0.2.0 entry — returns success on first attempt:

```
$ for i in $(seq 1 30); do
if curl -sf "https://crates.io/api/v1/crates/axonflow-sdk-rust/0.2.0" > /dev/null; then
echo "✅ axonflow-sdk-rust@0.2.0 is live on crates.io (after ${i} attempt(s))"
break
fi
sleep 15
done
✅ axonflow-sdk-rust@0.2.0 is live on crates.io (after 1 attempt(s))
```

Self-review

• All 4 references to the old timing (comment, loop bound, success message, error message) updated consistently — `grep -nE "12|3 minutes" .github/workflows/release.yml` returns nothing matching the old values.
• YAML still parses (`python3 -c 'yaml.safe_load(...)'`).
• Bash block in the `run:` step has valid syntax (`bash -n`).

CHANGELOG.md / Cargo.toml / source code unchanged.