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

0 files changed, 0 insertions, 0 deletions