feat: add host option to override session QR host

Add an optional `host` field to the base session request, mirroring
irmago's `host` field (irmago v0.14.0). It overrides the host used in
the QR (`Qr.u`) and is validated server-side against the requestor's
configured `host_perms` allowlist.

The field serializes as `host` only when set, and a `.host(...)` builder
method is exposed on the disclosure, signature, and issuance builders.

Closes #7

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dobby-coder[bot]

src/sessionrequest.rs

@@ -138,6 +138,11 @@ pub struct BaseRequest {
         deserialize_with = "crate::util::de_int_key"
     )]
     pub labels: HashMap<usize, TranslatedString>,
+    /// Host to use in the session QR (`Qr.u`), overriding the IRMA server's default.
+    /// The server validates this against the requestor's configured `host_perms` allowlist.
+    /// Requires irmago v0.14.0 or newer on the server side.
+    #[serde(skip_serializing_if = "Option::is_none", default)]
+    pub host: Option<String>,
 }
 
 /// IRMA session requests
@@ -179,6 +184,7 @@ impl BaseRequestBuilder {
                 return_url: None,
                 augment_return: false,
                 labels: HashMap::new(),
+                host: None,
             },
         }
     }
@@ -215,6 +221,11 @@ impl BaseRequestBuilder {
         self.base.return_url = Some(return_url);
         self.base.augment_return = true;
     }
+
+    fn host(&mut self, host: String) {
+        debug_assert!(self.base.host.is_none());
+        self.base.host = Some(host);
+    }
 }
 
 impl Default for BaseRequestBuilder {
@@ -276,6 +287,14 @@ impl DisclosureRequestBuilder {
         self.base.augmented_return_url(return_url);
         self
     }
+
+    /// Set the host to use in the session QR, overriding the IRMA server's default.
+    /// The server validates this against the requestor's configured `host_perms` allowlist
+    /// (requires irmago v0.14.0 or newer).
+    pub fn host(mut self, host: String) -> DisclosureRequestBuilder {
+        self.base.host(host);
+        self
+    }
 }
 
 /// Build a signature request
@@ -335,6 +354,14 @@ impl SignatureRequestBuilder {
         self.base.augmented_return_url(return_url);
         self
     }
+
+    /// Set the host to use in the session QR, overriding the IRMA server's default.
+    /// The server validates this against the requestor's configured `host_perms` allowlist
+    /// (requires irmago v0.14.0 or newer).
+    pub fn host(mut self, host: String) -> SignatureRequestBuilder {
+        self.base.host(host);
+        self
+    }
 }
 
 /// Build a request to issue one or more credentials
@@ -401,6 +428,14 @@ impl IssuanceRequestBuilder {
         self.base.augmented_return_url(return_url);
         self
     }
+
+    /// Set the host to use in the session QR, overriding the IRMA server's default.
+    /// The server validates this against the requestor's configured `host_perms` allowlist
+    /// (requires irmago v0.14.0 or newer).
+    pub fn host(mut self, host: String) -> IssuanceRequestBuilder {
+        self.base.host(host);
+        self
+    }
 }
 
 /// An IRMA request extended with extra information for the server on how to execute it.
@@ -543,6 +578,53 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_host_request() {
+        // host is omitted from the serialized request when not set
+        let req1 = DisclosureRequestBuilder::new()
+            .add_discon(vec![vec![AttributeRequest::Simple("a.b.c.d".into())]])
+            .build();
+        assert!(!serde_json::to_string(&req1).unwrap().contains("host"));
+
+        // host is serialized as "host" when set, and survives a round-trip
+        let req2 = DisclosureRequestBuilder::new()
+            .add_discon(vec![vec![AttributeRequest::Simple("a.b.c.d".into())]])
+            .host("https://example.com".into())
+            .build();
+        assert_eq!("{\"@context\":\"https://irma.app/ld/request/disclosure/v2\",\"disclose\":[[[\"a.b.c.d\"]]],\"host\":\"https://example.com\"}", serde_json::to_string(&req2).unwrap());
+        assert_eq!(
+            req2,
+            serde_json::from_str(&serde_json::to_string(&req2).unwrap()).unwrap()
+        );
+
+        // the host setter is available on every public builder
+        let req3 = SignatureRequestBuilder::new("testmessage".into())
+            .add_discon(vec![vec![AttributeRequest::Simple("a.b.c.d".into())]])
+            .host("https://signature.example.com".into())
+            .build();
+        assert_eq!("{\"@context\":\"https://irma.app/ld/request/signature/v2\",\"message\":\"testmessage\",\"disclose\":[[[\"a.b.c.d\"]]],\"host\":\"https://signature.example.com\"}", serde_json::to_string(&req3).unwrap());
+        assert_eq!(
+            req3,
+            serde_json::from_str(&serde_json::to_string(&req3).unwrap()).unwrap()
+        );
+
+        let req4 = IssuanceRequestBuilder::new()
+            .add_credential(Credential {
+                credential: "a.b.c".into(),
+                validity: Some(123456789),
+                attributes: hashmap![
+                    "d".into() => "e".into(),
+                ],
+            })
+            .host("https://issuance.example.com".into())
+            .build();
+        assert_eq!("{\"@context\":\"https://irma.app/ld/request/issuance/v2\",\"credentials\":[{\"credential\":\"a.b.c\",\"validity\":123456789,\"attributes\":{\"d\":\"e\"}}],\"host\":\"https://issuance.example.com\"}", serde_json::to_string(&req4).unwrap());
+        assert_eq!(
+            req4,
+            serde_json::from_str(&serde_json::to_string(&req4).unwrap()).unwrap()
+        );
+    }
+
     #[test]
     fn test_signature_request() {
         let req1 = SignatureRequestBuilder::new("testmessage".into())