chlog: number Ubuntu backports with the per-release SRU scheme

--backport was Debian-only: ~bpo is backports.debian.org's scheme and
its number the Debian release, so Ubuntu targets were rejected outright.
Ubuntu backports have their own documented scheme (Ubuntu version-
strings): the development release's version with a per-release ~YY.MM.1
appended, sorting before it (3.1-1ubuntu2 backported to 22.04 becomes
3.1-1ubuntu2~22.04.1; native 3.1 becomes 3.1~22.04.1) and independent of
the version the target release carries. The .N increments for
subsequent per-release SRU uploads.

backport_series_number becomes backport_suffix_for_series: the release
number comes from the new generic get_series_release_number (version
column of the target series' own distro-info data, leading token kept —
"12" for bookworm, "26.04" out of resolute's "26.04 LTS"; empty column
as on sid/experimental means None), and the suffix is picked per vendor:
~bpoNN+ for Debian (plain integer releases only), ~YY.MM. for Ubuntu.
Unnumbered series still error before anything is written.

Also serialize the changelog tests that mutate the process-global
DEBFULLNAME/DEBEMAIL variables behind a tokio Mutex: run in parallel
they raced each other's identity reads, which started failing
intermittently as generate_entry tests accumulated.

src/distro_info.rs

@@ -712,6 +712,30 @@ pub async fn get_debian_series_number(series: &str) -> Result<Option<String>, Bo
     Ok(None)
 }
 
+/// The release number of a distribution series, paired with the dist it
+/// belongs to: the version column of the series data, stripped to its
+/// leading token ("12" for Debian bookworm, "26.04" out of Ubuntu
+/// resolute's "26.04 LTS"). `None` when the series carries no version at
+/// all (Debian's rolling sid/experimental have an empty column;
+/// pseudo-versions like "unstable" pass through, callers validate per
+/// vendor). Errors when no known distribution carries the series.
+pub async fn get_series_release_number(
+    series: &str,
+) -> Result<Option<(String, String)>, Box<dyn Error>> {
+    let dist = get_dist_from_series(series).await?;
+    for info in get_ordered_series(&dist).await? {
+        if info.series == series {
+            let number = info
+                .version
+                .as_deref()
+                .and_then(|version| version.split_whitespace().next())
+                .map(str::to_string);
+            return Ok(number.map(|number| (dist, number)));
+        }
+    }
+    Ok(None)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -1033,6 +1057,25 @@ mod tests {
         assert!(unknown_number.is_none());
     }
 
+    #[tokio::test]
+    async fn test_get_series_release_number() {
+        let (dist, bookworm) = get_series_release_number("bookworm")
+            .await
+            .unwrap()
+            .unwrap();
+        assert_eq!(dist, "debian");
+        assert_eq!(bookworm, "12");
+
+        // Ubuntu LTS rows carry a " LTS" decoration: only the leading
+        // YY.MM token is the release number
+        let (dist, noble) = get_series_release_number("noble").await.unwrap().unwrap();
+        assert_eq!(dist, "ubuntu");
+        assert_eq!(noble, "24.04");
+
+        // No known dist carries the series
+        assert!(get_series_release_number("not-a-series").await.is_err());
+    }
+
     #[tokio::test]
     async fn test_get_keyring_urls_sid() {
         // Test that 'sid' returns keyrings from the 3 latest released versions