Switch httpclient default back-end over to libcurl (#832)

Switches the default HTTP client to the libcurl-based backend and follows up with a series of correctness fixes and code quality improvements to `CurlHttpClient`.

**Backend switch & build fixes:**
- Switch default HTTP client to libcurl-based backend
- Suppress `[[nodiscard]]` warning when building fmt
- Miscellaneous bugfixes in HttpClient/libcurl
- Pass `-y` to `xmake config` in `xmake test` task

**Boilerplate reduction:**
- Add `Session::SetHeaders()` for RAII ownership of `curl_slist`, eliminating manual `curl_slist_free_all` calls from every verb method
- Add `Session::PerformWithResponseCallbacks()` to absorb the repeated 12-line write+header callback setup block
- Extract `ParseHeaderLine()` shared helper, replacing 4 duplicate header-parsing implementations
- Extract `BuildHeaderMap()` and `ApplyContentTypeFromHeaders()` helpers to deduplicate header-to-map conversion and Content-Type scanning
- Unify the two `DoWithRetry` overloads (PayloadFile variant now delegates to the Validate variant)

**Correctness fixes:**
- `TransactPackage`: both phases now use `PerformWithResponseCallbacks()`, fixing missing abort support and a dead header collection loop
- `TransactPackage`: error path now routes through `CommonResponse`, preserving curl error codes and messages for the caller
- `ValidatePayload`: merged 3 separate header-scan loops into a single pass

**Performance improvements:**
- Replace `fmt::format` with `ExtendableStringBuilder` in `BuildHeaderList` and `BuildUrlWithParameters`, eliminating heap allocations in the common case
- Replace `curl_easy_escape`/`curl_free` with inline URL percent-encoding using `AsciiSet`
- Remove wasteful `CommonResponse(...)` construction in retry logging, formatting directly from `CurlResult` fields

1 files changed, 1 insertions, 1 deletions

diff --git a/xmake.lua b/xmake.lua
index e9bed93aa..94343cc90 100644
--- a/xmake.lua
+++ b/xmake.lua
@@ -519,7 +519,7 @@ task("test")
         if config.get("mode") ~= "debug" or config.get("plat") ~= plat or config.get("arch") ~= arch then
             local toolchain_flag = config.get("toolchain") and ("--toolchain=" .. config.get("toolchain")) or ""
             local sdk_flag = config.get("sdk") and ("--sdk=" .. config.get("sdk")) or ""
-            os.exec("xmake config -c -m debug -p %s -a %s %s %s", plat, arch, toolchain_flag, sdk_flag)
+            os.exec("xmake config -y -c -m debug -p %s -a %s %s %s", plat, arch, toolchain_flag, sdk_flag)
         end
 
         -- Build targets we're going to run