multi: probe for IPv6 functionality in multi_init()

[bagder]

In some legacy systems IPv6 might dynamically work/not work and thus curl needs to check/probe to see if it should indeed be used.

This change moves the probe that checks for working IPv6 to the multi handle setup function instead of delaying it to when the first name resolve is performed. This avoids a later tricky error path if the socket cannot be created due to OOM.

[bagder]

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[augmentcode]

🤖 Augment PR Summary

Summary: Move IPv6 capability probing to multi handle initialization so the result is known upfront.
Motivation: Avoid a late resolver-time probe that can fail in hard-to-handle paths (e.g., socket() OOM).
Changes:

• Add `Curl_probeipv6()` and store the result on `Curl_multi` as `ipv6_works`.
• Simplify `Curl_ipv6works()` to consult the cached multi-handle flag and remove the old `IPV6_*` tri-state.
• Run the probe in `Curl_multi_handle()` so multi creation fails early on OOM.

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[augmentcode]

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[Copilot]

Pull request overview

This PR moves the IPv6 functionality probe from lazy initialization during name resolution to eager initialization during multi handle setup. This prevents potential out-of-memory errors in a complex error path later when DNS resolution happens.

Changes:

• Replaced tri-state IPv6 status tracking (UNKNOWN/DEAD/WORKS) with a simple boolean flag
• Moved IPv6 probing from Curl_ipv6works() to new Curl_probeipv6() function called during Curl_multi_handle() initialization
• Simplified Curl_ipv6works() to just return the pre-computed status

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
lib/multihandle.h	Changed ipv6_up from unsigned char with three states to a simple BIT(ipv6_works) boolean flag
lib/multi.c	Added call to Curl_probeipv6() during multi handle initialization with proper error handling
lib/hostip.h	Added Curl_probeipv6() function declaration for USE_IPV6, defined as CURLE_OK macro otherwise
lib/hostip.c	Implemented Curl_probeipv6() with socket creation test and OOM detection; simplified Curl_ipv6works() to return cached result

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