feat: make AI Moderation provider-agnostic

[Alam-2U]

Description

AI moderation was hard-wired to XPert. The service built the XPert request and parsed the XPert response itself, so the provider and the moderation workflow were one piece of code — there was no way to run this against anything else.

This separates them. Forum now defines a moderation interface and ships no provider of its own. AIModerationService orchestrates only — deduplication, caching, spam flagging, soft deletion, audit logging — and delegates classification to whichever backend AI_MODERATION_BACKEND names. XPert becomes one implementation of that interface, chosen by configuration rather than by being the only option.

No behaviour change for edX: XPert is selected in edx-internal and sends exactly the requests it sent before.

Changes

• A provider-agnostic backend interface, plus shared handling for HTTP classifiers (endpoint, timeouts, error handling, response parsing). The generic layer holds no credential and sets no auth header — providers disagree about auth more than anything else, so that belongs to each backend.
• XPert moved behind that interface, its request and response handling unchanged.
• Backend selected by AI_MODERATION_BACKEND and loaded by dotted path.
• A default prompt and timeouts ship with forum, so standing up a backend doesn't also mean writing a spam-classifier prompt.
• Settings declared in forum's own plugin settings, deferring to whatever the deployment already configured. No edx-platform change is needed.
• Provider failure and misconfiguration degrade moderation and log the reason; neither can block a learner from posting.
• A how-to for operators writing their own backend.

Ticket ref

LP-1032

Related PR

• edx-internal: <link>

[santhosh-apphelix-2u]

One small thing to confirm: since forum.ai_moderation is being moved from a single module to a package structure, please make sure there are no remaining imports using the old path in any other repo or downstream branch.

[Alam-2U]

@santhosh-apphelix-2u I have confirmed there is no remaining imports using the old path in any other repo or downstream branch