Avoid redundant curl connections during telemetry HTTP sends

[bmehta001]

Why

A single telemetry HTTP send currently performs CURLOPT_CONNECT_ONLY and then the actual HTTP transfer. libcurl documents that connect-only connections cannot be reused; a loopback regression against curl 8.22.0 observed two TCP accepts for one GET before this change. The readiness poll therefore checks a different connection from the one carrying the request.

What changed

• With curl 7.80+, use CURLOPT_PREREQFUNCTION to dispatch OnSending after connection establishment and before the HTTP request, within one transfer. Preserve connect/send failure events and abort on cancellation or callback exceptions without unwinding through libcurl.
• Choose the connect-only fallback using the loaded libcurl version, even when built with newer headers; preserve its state-event timing on runtimes older than 7.80. The one-connection behavior applies only to 7.80+ runtimes.
• Cover one connection for GET and binary POST, state-event order, cancellation from OnSending, and exceptions raised by an OnSending observer.

OnSending is now delivered from a libcurl callback on curl 7.80+, rather than between two transfers. On curl 7.80+, OnSending runs inside libcurl and receives a null handle to prevent in-transfer option changes; configure curl on the OnConnecting event instead. Older curl retains the legacy event timing and handle.

Additional curl safety fixes

• Propagate curl_global_init failures as creation/local failures without calling other curl APIs after failed initialization; keep the handle null for safe teardown.
• Convert response-vector allocation exceptions to a short write (CURLE_WRITE_ERROR) rather than unwinding through libcurl's C callback, when C++ exceptions are enabled. No-exceptions builds also compile.
• Wait for the loopback server to record an accepted connection when an OnSending observer aborts the transfer immediately.

Validation

• curl 8.22.0 ASan/UBSan Linux: 18 focused tests x 10 iterations (180 executions), with leak checking and no skips or sanitizer reports.
• Linux: curl transport compiles with -fno-exceptions against curl 8.22.0 headers.
• Windows (Visual Studio 2026): 59 targeted HTTP tests passed.
• Linux (system curl 8.5.0): 32 focused curl tests passed; 2 GET/POST tests also passed with a temporary runtime-version shim reporting 7.79 to exercise the legacy path.
• git diff --check passed.

This fixes the demonstrated extra connection, not a proven root cause of the reported Curl_wildcard_dtor SIGSEGV. The crash report has no core, faulting instruction, registers, or loaded-module map to establish that cause.

[Copilot]

Copilot review overview

🟡 Changes recommended

Runtime-version fallback and callback reentrancy/state-event issues remain unresolved.

Review effort: Balanced
Findings: 1

Open (1)

• Runtime libcurl version can break legacy fallback · New

What changed in this PR

Updates the curl transport to avoid redundant connections while preserving telemetry state events and improving failure safety.

Changes:

• Uses CURLOPT_PREREQFUNCTION for modern libcurl.
• Handles global initialization and callback allocation failures.
• Adds connection, cancellation, binary POST, and exception tests.

File	Description
lib/​http/​HttpClient_Curl.hpp	Implements connection-ready callbacks and safety handling.
lib/​http/​HttpClient_Curl.cpp	Propagates global initialization failures.
tests/​unittests/​HttpClientCurlTests.cpp	Adds transport regression coverage.

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[Copilot]

Copilot review overview

🟡 Changes recommended

Existing listeners can now call curl_easy_setopt reentrantly on an active handle from OnSending, causing unsupported behavior.

Review effort: Balanced
Findings: 1

Open (1)

• OnSending exposes active CURL handle for unsafe in-transfer mutation · New

Resolved since last review (1)

• Runtime libcurl version can break legacy fallback

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation preserves older-runtime behavior and adds focused coverage for connection reuse, callback ordering, cancellation, and failure paths.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• OnSending exposes active CURL handle for unsafe in-transfer mutation