lift main.rs business logic into the library

The chlog target-series resolution becomes changelog::series_candidates
(UNRELEASED pinning, development-series default and fallbacks modeled
by SeriesCandidates), PPA references get package_info::split_ppa
(shared by pull and deb, now also rejecting empty parts), and SSH
endpoints get context::ContextConfig::from_endpoint — so a library
consumer can resolve series, validate PPAs and build context
configurations without reimplementing the CLI's rules. All three carry
unit tests; main.rs keeps only parsing of flags and error handling.

src/context/api.rs

@@ -117,6 +117,28 @@ pub enum ContextConfig {
     },
 }
 
+impl ContextConfig {
+    /// Build an SSH context configuration from an endpoint of the form
+    /// `[ssh://][user@]host[:port]`.
+    pub fn from_endpoint(endpoint: &str) -> Result<Self, String> {
+        let re = regex::Regex::new(
+            r"^(?:ssh://)?(?:(?P<user>[^@]+)@)?(?P<host>[^:/]+)(?::(?P<port>\d+))?$",
+        )
+        .expect("valid endpoint regex");
+        let cap = re.captures(endpoint).ok_or_else(|| {
+            format!("Invalid endpoint format: '{endpoint}'. Expected [ssh://][user@]host[:port]")
+        })?;
+        let host = cap.name("host").unwrap().as_str().to_string();
+        let user = cap.name("user").map(|m| m.as_str().to_string());
+        let port = cap
+            .name("port")
+            .map(|m| m.as_str().parse::<u16>())
+            .transpose()
+            .map_err(|_| "Invalid port number".to_string())?;
+        Ok(ContextConfig::Ssh { host, user, port })
+    }
+}
+
 /// A context, allowing to run commands, read and write files, etc
 pub struct Context {
     /// Configuration for the context
@@ -455,3 +477,57 @@ fn contextualize_spawn_error(program: &str, e: io::Error) -> io::Error {
         io::Error::new(e.kind(), format!("Could not run '{program}': {e}"))
     }
 }
+
+#[cfg(test)]
+mod endpoint_tests {
+    use super::*;
+
+    /// Every accepted endpoint spelling maps to the expected config.
+    #[test]
+    fn from_endpoint_parses_all_spellings() {
+        assert_eq!(
+            ContextConfig::from_endpoint("myhost"),
+            Ok(ContextConfig::Ssh {
+                host: "myhost".into(),
+                user: None,
+                port: None,
+            })
+        );
+        assert_eq!(
+            ContextConfig::from_endpoint("admin@myhost"),
+            Ok(ContextConfig::Ssh {
+                host: "myhost".into(),
+                user: Some("admin".into()),
+                port: None,
+            })
+        );
+        assert_eq!(
+            ContextConfig::from_endpoint("myhost:2222"),
+            Ok(ContextConfig::Ssh {
+                host: "myhost".into(),
+                user: None,
+                port: Some(2222),
+            })
+        );
+        assert_eq!(
+            ContextConfig::from_endpoint("ssh://admin@myhost:22"),
+            Ok(ContextConfig::Ssh {
+                host: "myhost".into(),
+                user: Some("admin".into()),
+                port: Some(22),
+            })
+        );
+    }
+
+    /// Non-numeric ports and extra segments are format errors; a
+    /// non-u16 numeric port is a port error.
+    #[test]
+    fn from_endpoint_rejects_malformed_endpoints() {
+        for bad in ["", "a/b/c", "host:notaport"] {
+            let err = ContextConfig::from_endpoint(bad).unwrap_err();
+            assert!(err.contains("Invalid endpoint format"), "{err}");
+        }
+        let err = ContextConfig::from_endpoint("host:99999").unwrap_err();
+        assert_eq!(err, "Invalid port number");
+    }
+}