fix: fallback to HTTP/2 on HTTP3 DNS error

[barjin]

Makes DNS client in HTTP/3 record resolution optional. If the initial connection fails with Error, impit will return false for every call to host_supports_h3 (unless, e.g. alt-svc header has been registered for this domain).

[Copilot]

Pull Request Overview

This PR adds error handling for DNS client initialization in HTTP/3 record resolution, allowing the system to gracefully fallback to HTTP/2 when DNS errors occur.

• Makes DNS client optional in the H3Engine struct to handle initialization failures
• Adds graceful error handling when DNS client creation fails
• Modifies host_supports_h3 to return false when DNS client is unavailable

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