feat(rust): Add Config::header() for custom HTTP and WebSocket headers (#520)

## Summary

- Added `custom_headers: HashMap<String, String>` field to `Config`
struct
- Added `Config::header(key, value) -> Self` builder method to set
custom headers
- Custom headers are injected into every HTTP request (via
`create_http_client()`) and WebSocket upgrade request (via
`create_ws_request()`)
- Updated `CHANGELOG.md`

## Use Case

Enables callers to pass arbitrary headers (e.g. `x-cli-cmd`,
`x-channel-key`) for server-side telemetry and analytics without
modifying the SDK internals.

```rust
let config = Config::from_oauth(oauth)
    .header("x-cli-cmd", "cash-flow")
    .header("x-channel-key", "kol-123");
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: huacnlee

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Added
 
+- **Rust:** `Config::header(key, value)` builder method to inject custom headers into every HTTP request and WebSocket upgrade request.
 - **Rust, Python:** `ContentContext` adds three new methods:
   - `topic_detail(topic_id)` — get detail of a single topic.
   - `list_topic_replies(opts)` — list replies for a topic, with optional page/size filtering.

rust/src/config.rs

@@ -6,7 +6,7 @@ use std::{
     sync::Arc,
 };
 
-pub(crate) use http::{HeaderValue, Request, header};
+pub(crate) use http::{HeaderName, HeaderValue, Request, header};
 use longbridge_httpcli::{HttpClient, HttpClientConfig, is_cn};
 use longbridge_oauth::OAuth;
 use num_enum::IntoPrimitive;
@@ -127,6 +127,8 @@ pub struct Config {
     pub(crate) enable_print_quote_packages: bool,
     pub(crate) language: Language,
     pub(crate) log_path: Option<PathBuf>,
+    /// Extra headers injected into every HTTP and WebSocket upgrade request.
+    pub(crate) custom_headers: HashMap<String, String>,
 }
 
 /// Reads an env var by trying `LONGBRIDGE_<suffix>` first, then falling back
@@ -218,6 +220,7 @@ impl Config {
             push_candlestick_mode: extras.push_candlestick_mode,
             enable_print_quote_packages: extras.enable_print_quote_packages,
             log_path: extras.log_path,
+            custom_headers: Default::default(),
         }
     }
 
@@ -266,6 +269,7 @@ impl Config {
             push_candlestick_mode: extras.push_candlestick_mode,
             enable_print_quote_packages: extras.enable_print_quote_packages,
             log_path: extras.log_path,
+            custom_headers: Default::default(),
         }
     }
 
@@ -320,6 +324,7 @@ impl Config {
             push_candlestick_mode: extras.push_candlestick_mode,
             enable_print_quote_packages: extras.enable_print_quote_packages,
             log_path: extras.log_path,
+            custom_headers: Default::default(),
         })
     }
 
@@ -419,7 +424,12 @@ impl Config {
             config = config.http_url(url.clone());
         }
 
-        HttpClient::new(config).header(header::ACCEPT_LANGUAGE, self.language.as_str())
+        let mut client =
+            HttpClient::new(config).header(header::ACCEPT_LANGUAGE, self.language.as_str());
+        for (key, value) in &self.custom_headers {
+            client = client.header(key.as_str(), value.as_str());
+        }
+        client
     }
 
     fn create_ws_request(&self, url: &str) -> tokio_tungstenite::tungstenite::Result<Request<()>> {
@@ -428,6 +438,14 @@ impl Config {
             header::ACCEPT_LANGUAGE,
             HeaderValue::from_str(self.language.as_str()).unwrap(),
         );
+        for (key, value) in &self.custom_headers {
+            if let (Ok(name), Ok(val)) = (
+                HeaderName::from_bytes(key.as_bytes()),
+                HeaderValue::from_str(value),
+            ) {
+                request.headers_mut().append(name, val);
+            }
+        }
         Ok(request)
     }
 
@@ -471,6 +489,13 @@ impl Config {
         self
     }
 
+    /// Add a custom header to every HTTP request and WebSocket upgrade request.
+    #[must_use]
+    pub fn header(mut self, key: impl Into<String>, value: impl Into<String>) -> Self {
+        self.custom_headers.insert(key.into(), value.into());
+        self
+    }
+
     /// Set the HTTP endpoint URL in place.
     pub fn set_http_url(&mut self, url: impl Into<String>) {
         self.http_url = Some(url.into());