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, 11 insertions, 0 deletions

diff --git a/src/zencore/string.cpp b/src/zencore/string.cpp
index ed0ba6f46..358722b0b 100644
--- a/src/zencore/string.cpp
+++ b/src/zencore/string.cpp
@@ -1181,6 +1181,17 @@ TEST_CASE("string")
 		CHECK(StrCaseCompare("BBr", "Bar", 2) > 0);
 	}
 
+	SUBCASE("StrCaseCompare")
+	{
+		CHECK(StrCaseCompare("foo"sv, "FoO"sv) == 0);
+		CHECK(StrCaseCompare("foo"sv, "FoOz"sv) < 0);
+		CHECK(StrCaseCompare("fooo"sv, "FoO"sv) > 0);
+		CHECK(StrCaseCompare("Bar"sv, "bAs"sv) < 0);
+		CHECK(StrCaseCompare("bAr"sv, "Bas"sv) < 0);
+		CHECK(StrCaseCompare("BBr"sv, "Bar"sv) > 0);
+		CHECK(StrCaseCompare("Bbr"sv, "BAr"sv) > 0);
+	}
+
 	SUBCASE("ForEachStrTok")
 	{
 		const auto		   Tokens			  = "here,is,my,different,tokens"sv;