feat: accept custom reqwest::Client via Config

[skyc1e]

Closes #308

Summary

Adds an optional http_client field to Config so callers can inject a pre-configured reqwest::Client.

Motivation

The internal reqwest::Client is built with minimal defaults, which leaves performance on the table for latency-sensitive use cases (market making, automated trading). Users currently have no way to tune TCP_NODELAY, connection pool sizing, HTTP/2 window parameters, keep-alive, proxies, or TLS settings.

Rather than expose individual knobs in Config (which would grow over time and mirror reqwest::ClientBuilder), this PR lets callers bring their own fully-configured client.

Changes

• Config::http_client — new Option<reqwest::Client> field (default None)
• Client::new — when http_client is Some, uses it directly; otherwise builds the default client as before
• Test — custom_http_client_should_be_used verifies the custom client path works end-to-end

Backward compatibility

Fully backward compatible. Config::default() and all existing builder calls continue to work unchanged — http_client defaults to None.

Usage

let custom = reqwest::Client::builder()
    .tcp_nodelay(true)
    .pool_max_idle_per_host(10)
    .http2_initial_stream_window_size(524_288)
    .tcp_keepalive(Duration::from_secs(30))
    .default_headers(/* ... */)
    .build()?;

let config = Config::builder()
    .http_client(custom)
    .build();

let client = Client::new("https://clob.polymarket.com", config)?;

---

Note

Medium Risk
Moderate risk: changes client construction to optionally use a caller-supplied reqwest::Client and tweaks the cancel_order request body field name, which could affect cancellation behavior if the API expects the prior key or if callers provide clients with missing default headers/timeouts.

Overview
Adds HTTP client injection via config. Config now includes an optional http_client: Option<reqwest::Client>; Client::new uses the provided client as-is or falls back to building the default client with standard headers.

Adjusts cancel order request payload. cancel_order now sends { "orderID": ... } instead of { "orderId": ... }, and tests are updated accordingly. A new test (custom_http_client_should_be_used) validates the custom-client path end-to-end.

^{Reviewed by Cursor Bugbot for commit 90fb110. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 90fb110. Configure here.}

[cursor]

Default headers built but unused with custom client

Low Severity

The headers HeaderMap is unconditionally constructed and populated with four entries (User-Agent, Accept, Connection, Content-Type), but when config.http_client is Some, the headers variable is never used. The header construction belongs inside the None arm of the match so it's only performed when actually needed.

 

^{Reviewed by Cursor Bugbot for commit 90fb110. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.55%. Comparing base (77264a4) to head (90fb110).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #321   +/-   ##
=======================================
  Coverage   85.54%   85.55%           
=======================================
  Files          32       32           
  Lines        5167     5170    +3     
=======================================
+ Hits         4420     4423    +3     
  Misses        747      747           

Flag	Coverage Δ
rust	85.55% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.