Consider setting `min_value_msat` in LSPS2's `create_inbound_payment`

[tnull]

While we still make sure to only claim if the withheld amount is what we expect, I think we should be able to set the min_value_msat field on create_inbound_payment to the net amount, which probably would yet another layer of protection?

[Jolah1]

@tnull
I'd like to pick this up.

Proposed fix:
In lsps2_create_jit_invoice (client/lsps2.rs), both create_inbound_payment_for_hash and create_inbound_payment pass None for min_value_msat. I'd set it to the net amount — amount_msat − max_total_opening_fee_msat — so LDK floors what it'll claim, on top of the existing withheld-amount check (using the max total fee gives the lowest acceptable net, the right floor). This only works where the net is deterministic — fixed amount_msat and fixed max_total_opening_fee_msat; for variable-amount or proportional-fee-only quotes I'd leave it None.

One sequencing question: the BOLT12 receive-through-JIT path in #817 is another inbound-payment site that wants the same floor. Would you rather I (a) do the BOLT11 paths now and follow up on BOLT12 once #817 lands, or (b) wait and cover both in one PR? Either way is fine, depends on your read of #817's timeline.

And does amount_msat − max_total_opening_fee_msat match the net figure you had in mind?

[Jolah1]

Update: #817 has been superseded by #999. I checked the current head, and its BOLT11 LSPS2 invoice registration still passes None for min_value_msat, so this issue remains applicable. Since #999 substantially rewrites this path, I’ll hold off unless you’d prefer this handled on top of #999 or as a follow-up after it lands.