fix: Address critical issues in client configuration registry implementation

- Fixed HeaderMapInit parsing bug that would cause compilation errors
- Added comprehensive documentation with usage examples and DSL reference
- Improved error handling with better error messages for invalid headers
- Added test coverage for both macro and registry functionality
- Added debug inspection capabilities for registered configurations
- Fixed module name conflicts in tests by using separate modules

All tests now passing:
- 7 macro tests validating DSL parsing and code generation
- 4 registry tests verifying configuration collection and application

common/http-api-client/src/lib.rs

@@ -136,10 +136,10 @@
 //! ```
 #![warn(missing_docs)]
 
+pub use inventory;
+pub use reqwest;
 pub use reqwest::ClientBuilder as ReqwestClientBuilder;
 pub use reqwest::StatusCode;
-pub use reqwest;
-pub use inventory;
 
 pub mod registry;
 
@@ -181,10 +181,10 @@ mod path;
 pub use dns::{HickoryDnsError, HickoryDnsResolver};
 
 // helper for generating user agent based on binary information
+use crate::registry::default_builder;
 #[doc(hidden)]
 pub use nym_bin_common::bin_info;
 use nym_http_api_client_macro::client_defaults;
-use crate::registry::default_builder;
 
 client_defaults!(
     priority = -100;

common/http-api-client/src/registry.rs

@@ -1,6 +1,14 @@
-/// A record contributed by any crate describing how to mutate a reqwest::ClientBuilder.
+//! Global registry for HTTP client configurations.
+//!
+//! This module provides a compile-time registry system that allows any crate
+//! in the workspace to contribute configuration modifications to HTTP clients.
+
 use crate::ReqwestClientBuilder;
 
+/// A configuration record that modifies a `ReqwestClientBuilder`.
+///
+/// Records are collected at compile-time via the `inventory` crate and
+/// applied in priority order when building HTTP clients.
 pub struct ConfigRecord {
     /// Lower numbers run earlier.
     pub priority: i32,
@@ -10,18 +18,36 @@ pub struct ConfigRecord {
 
 inventory::collect!(ConfigRecord);
 
+/// Returns the default builder with all registered configurations applied.
 pub fn default_builder() -> ReqwestClientBuilder {
     let mut b = ReqwestClientBuilder::new();
     let mut records: Vec<&'static ConfigRecord> =
-        inventory::iter::<ConfigRecord>
-            .into_iter()
-            .collect();
+        inventory::iter::<ConfigRecord>.into_iter().collect();
     records.sort_by_key(|r| r.priority); // lower runs first
     for r in records {
         b = (r.apply)(b);
     }
     b
 }
+
+/// Builds a client using the default builder with all registered configurations.
 pub fn build_client() -> reqwest::Result<reqwest::Client> {
     default_builder().build()
 }
+
+/// Debug function to inspect registered configurations.
+/// Returns a vector of (priority, function_pointer) tuples for debugging.
+#[cfg(debug_assertions)]
+pub fn inspect_registered_configs() -> Vec<(i32, usize)> {
+    let mut configs: Vec<(i32, usize)> = inventory::iter::<ConfigRecord>
+        .into_iter()
+        .map(|record| (record.priority, record.apply as usize))
+        .collect();
+    configs.sort_by_key(|(priority, _)| *priority);
+    configs
+}
+
+/// Returns the count of registered configuration records.
+pub fn registered_config_count() -> usize {
+    inventory::iter::<ConfigRecord>.into_iter().count()
+}

common/http-api-client/tests/registry_tests.rs

@@ -0,0 +1,70 @@
+use nym_http_api_client::registry;
+
+// Create separate modules to avoid name conflicts
+mod config_early {
+    use nym_http_api_client_macro::client_defaults;
+
+    client_defaults!(
+        priority = -200;
+        tcp_nodelay = true
+    );
+}
+
+mod config_late {
+    use nym_http_api_client_macro::client_defaults;
+
+    client_defaults!(
+        priority = 100;
+        pool_idle_timeout = std::time::Duration::from_secs(90)
+    );
+}
+
+#[test]
+fn test_registry_collects_configs() {
+    // Verify that configurations are being registered
+    let count = registry::registered_config_count();
+    // Should have at least the ones we registered above plus the default from lib.rs
+    assert!(
+        count >= 3,
+        "Expected at least 3 registered configs, got {}",
+        count
+    );
+}
+
+#[test]
+fn test_default_builder_applies_configs() {
+    // Test that default_builder returns a configured builder
+    let _builder = registry::default_builder();
+    // The builder should have all configurations applied
+    // We can't easily inspect the internals, but we verify it doesn't panic
+}
+
+#[test]
+fn test_build_client_works() {
+    // Test that we can successfully build a client with all configurations
+    let result = registry::build_client();
+    assert!(result.is_ok(), "Failed to build client: {:?}", result.err());
+}
+
+#[cfg(debug_assertions)]
+#[test]
+fn test_inspect_configs() {
+    // In debug mode, test that we can inspect registered configurations
+    let configs = registry::inspect_registered_configs();
+
+    // Verify configs are sorted by priority
+    for window in configs.windows(2) {
+        assert!(window[0].0 <= window[1].0, "Configs not sorted by priority");
+    }
+
+    // Verify we have configs at different priority levels
+    let priorities: Vec<i32> = configs.iter().map(|(p, _)| *p).collect();
+    assert!(
+        priorities.iter().any(|&p| p < 0),
+        "Expected negative priority configs"
+    );
+    assert!(
+        priorities.iter().any(|&p| p >= 0),
+        "Expected non-negative priority configs"
+    );
+}