feat: allow setting custom preimage in spontaneous-send

[f3r10]

address #226

Summary

• Adds an optional preimage field to SpontaneousSendRequest, allowing callers to supply a custom 32-byte hex preimage instead of having the node generate one randomly
• Dispatches to the appropriate ldk-node method (send_with_preimage or send_with_preimage_and_custom_tlvs) based on whether preimage and/or custom TLVs are present
• Exposes the flag in the CLI (--preimage) and MCP schema
• Adds an e2e test verifying the receiver observes a payment hash equal to
  SHA256 of the provided preimage

[ldk-reviews-bot]

👋 Thanks for assigning @benthecarman as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[benthecarman]

nit: can we have a new line between the imports and const

[Anyitechs]

@f3r10 Looks like you missed this in your latest push.

[benthecarman]

please add help text

[Anyitechs]

Thanks!

The CI is currently failing because the proto files are now out of date and needs to be updated.

[Anyitechs]

No strong opinion here, especially as I believe there's really no runtime cost in iterating and mapping over an empty vector. But since we're creating the vector and immediately checking if it's empty in the match statement below.

I'm wondering if we could do something like this instead?

let custom_tlvs: Option<Vec<CustomTlvRecord>> = if request.custom_tlvs.is_empty() {
  None
} else {
  Some(request.custom_tlvs.iter().map(proto_to_node_custom_tlv).collect())
};

let payment_id = match (preimage, custom_tlvs) {
		(None, None) => context.node.spontaneous_payment().send(
			request.amount_msat,
			node_id,
			route_parameters,
		)?,
		(None, Some(custom_tlvs)) => context.node.spontaneous_payment().send_with_custom_tlvs(
			request.amount_msat,
			node_id,
			route_parameters,
			custom_tlvs,
		)?,
		(Some(preimage), None) => context.node.spontaneous_payment().send_with_preimage(
			request.amount_msat,
			node_id,
			preimage,
			route_parameters,
		)?,
		(Some(preimage), Some(custom_tlvs)) => context
			.node
			.spontaneous_payment()
			.send_with_preimage_and_custom_tlvs(
				request.amount_msat, 
                 node_id, 
                 custom_tlvs, 
                 preimage, 
                 route_parameters,
			)?,
};

[Anyitechs]

@f3r10 Looks like you missed this in your latest push.