Fix dead links in ChatGPT responses (#1276)

[vlaym]

Closes #1276

What was done

• Integrated LinkDetection.replaceDeadLinks() (added in Add utilities to detect and replace broken links. #1366) into HelpSystemHelper#generateGptResponseEmbed(). The replacement runs before the embed is built, so both the auto-AI response in help threads and /chatgpt command are covered since they share the same embed builder.
• If the link check fails (e.g. network error), the original answer is kept as-is - no broken behavior.
• Added two unit tests in HelpSystemHelperTest: one verifying dead link replacement, one verifying fallback when replacement fails.

Approach

The issue proposes two options: ideally sending the response immediately and then editing it after async link checking, or as a fallback - replacing links before sending.

Current (fallback) flow:

post created → ChatGPT generates answer → dead links replaced → embed built → embed sent to Discord

Ideal flow (not implemented):

post created → ChatGPT generates answer → embed sent immediately → links checked in the background → already-sent message edited with clean links

The fallback was chosen because it keeps the change in one place (HelpSystemHelper) and is easy to test. The ideal flow would require mixing two different async mechanisms: checking links uses Java's CompletableFuture (because it makes plain HTTP requests to external sites — JDA is not involved here at all), while sending and editing Discord messages uses JDA's RestAction. These two do not compose directly, so some extra glue code is needed to chain them together correctly.