docs: clarify ProviderStateGenerator shorthand format in IntegrationJson.md; add warn log for missing value

Agent-Logs-Url: https://github.com/pact-foundation/pact-reference/sessions/fb2a6dd6-ef83-46ac-af88-f8edd5445fc0

Co-authored-by: mefellows <53900+mefellows@users.noreply.github.com>

Author: Copilot

rust/pact_ffi/IntegrationJson.md

@@ -35,7 +35,7 @@ pactffi_with_request(handle, "PUT", "{\"value\": \"/path/to/100\", \"pact:matche
 The `ProviderState` generator allows values to be sourced from provider state parameters during provider verification.
 There are two formats for specifying a `ProviderState` generator:
 
-### Shorthand format (expression only)
+### Shorthand format (expression only, no matching rule)
 
 When you only need a `ProviderState` generator (without any matching rule), you can use the shorthand format with just
 the `expression` and `value` keys:
@@ -47,8 +47,10 @@ the `expression` and `value` keys:
 }
 ```
 
-This sets up a `ProviderStateGenerator` with the given expression and uses the `value` as the example value. This
-shorthand is supported for query parameters, headers, path, and request/response bodies.
+This sets up a `ProviderStateGenerator` with the given expression and uses the `value` as the example value. **No
+matching rule is configured** — the example value is used as-is during consumer test execution, and the generator
+replaces it with the resolved provider state value during provider verification. This shorthand is supported for query
+parameters, headers, path, and request/response bodies.
 
 ### Full format (with matcher)
 

rust/pact_ffi/src/mock_server/bodies.rs

@@ -9,7 +9,7 @@ use lazy_static::lazy_static;
 use multipart_2021 as multipart;
 use regex::Regex;
 use serde_json::{Map, Value};
-use tracing::{debug, error, trace};
+use tracing::{debug, error, trace, warn};
 
 use pact_models::bodies::OptionalBody;
 use pact_models::content_types::ContentTypeHint;
@@ -73,7 +73,10 @@ pub fn process_object(
       let category = generator_category(matching_rules);
       generators.add_generator_with_subcategory(category, path.clone(), generator);
     }
-    obj.get("value").cloned().unwrap_or(Value::Null)
+    if !obj.contains_key("value") {
+      warn!("ProviderStateGenerator shorthand at path '{}' has no 'value' key; example value will be null", path);
+    }
+    obj.get("value").cloned().unwrap_or_default()
   } else {
     debug!("Configuring a normal object");
     Value::Object(obj.iter()

rust/pact_ffi/src/mock_server/handles.rs

@@ -1076,6 +1076,9 @@ fn from_integration_json_v2(
             let category = generator_category(matching_rules);
             generators.add_generator_with_subcategory(category, effective_path.clone(), generator);
           }
+          if !map.contains_key("value") {
+            warn!("ProviderStateGenerator shorthand at path '{}' has no 'value' key; example value will be null", path);
+          }
           map.get("value").cloned().unwrap_or_default()
         } else {
           debug!("Configuring a normal value using the 'value' attribute");