Use Halo HTTP security utilities for external fetches

[ruibaby]

Summary

• Upgrade the plugin to depend on Halo 2.25 and require Halo >=2.25.0 at runtime.
• Refactor external URL fetching to use Halos HttpSecurityUtils.secureHttpClient() and maxResponseSizeFilter(long).
• Remove plugin-local SSRF URL validation and redirect helper code.
• Replace implementation-detail tests with behavior-level coverage for link detail, RSS, and verification fetch behavior.

Why

Halo 2.25 exposes the HTTP security utilities through the plugin API, so the plugin can reuse Halos SSRF-safe HTTP client instead of maintaining duplicate address validation and socket handling logic.

Validation

• ./gradlew test
• ./gradlew build
• openspec validate use-halo-http-security-utils --strict
• git diff --check