fix(release): add non-blocking Maven Central propagation verify

[saurabhjain1592]

Summary

The release workflow reports "Deploy successful" the moment Sonatype Central validates the upload (per the v8.0.0 release run 25604825388 at 2026-05-09 15:37Z), then proceeds straight to creating the GitHub Release. But actual public availability on `repo1.maven.org/maven2` lags 10-30 minutes — operators couldn't tell whether the release was truly consumable.

Change

Add a polling step between `mvn deploy` and the GitHub Release that checks `repo1.maven.org` for the artifact .pom directly. Up to 30 min (60 × 30s).

`continue-on-error: true` keeps it non-blocking — if propagation exceeds 30 min, the GH release still fires and a clear warning points the operator at the Sonatype publish dashboard.

DoD (dryrun)

Ran the polling logic locally against two real versions:

```
$ # v7.1.0 (known propagated, prior release):
✅ axonflow-sdk:7.1.0 is live (after 1 attempt(s))

$ # v8.0.0 (just published, propagation in flight at deploy time):
✅ already propagated to Maven Central — https://repo1.maven.org/maven2/com/getaxonflow/axonflow-sdk/8.0.0/axonflow-sdk-8.0.0.pom
```

(v8.0.0 has now propagated since the original release run, ~25 min after `mvn deploy`.)

Self-review

• YAML still parses (`python3 -c 'yaml.safe_load(...)'`).
• Bash block in the `run:` step has valid syntax (`bash -n`).
• `POM_URL` host + path match `pom.xml` `com.getaxonflow` + `axonflow-sdk` exactly.
• Step inserted BEFORE GitHub Release creation so the release notes can reference availability state, but `continue-on-error` keeps the GH release un-blocked.

No source / pom.xml / artifact change.