[Snowbridge]: Ensure source always from AH for exported message

[yrong]

Resolves:
https://linear.app/snowfork/issue/SNO-1243
https://linear.app/snowfork/issue/SNO-1242

[acatangiu]

I think it's wrong to enforce this rule at the router level:

• maybe some other chain wants to use this router primitive with another bridge (unlikely)
• maybe we change the router in the AH runtime and the new one doesn't validate this bridge-specific rule (likely)
• XCM goes to bridge on Bridge Hub through another path than this router and this rule is not enforced (potentially dangerous)

This rule should definitely be enforced by the bridge itself on Bridge Hub. We can have the router also enforce it in order to fail early, fail at AH instead of failing on the next hop on BH. If this is the case, then the impl details comments below apply.

[acatangiu]

It is an anti-pattern to hardcode para-id constants in these types of primitives that could be in theory reused in other scenarios.

In practice this is fine, but the used pattern is to make this a generic parameter so it can be customized by the runtime.

[yrong]

Thanks. Yeah, just a workaround before getting some feedback.

Fixed in 277d51b

[yrong]

We can have the router also enforce it to fail early at AH instead of failing on the next hop on BH.

IIUC EthereumBlobExporter is the only entry point where users can send arbitrary messages to OutboundQueue. Meanwhile, since we've already patched with polkadot-fellows/runtimes#506 to disable the agent/channel, I'd assume it's safe for now, so maybe we can improve that in a separate PR?