Add trampoline fee override for outgoing payments

[ca-ruz]

Summary

This PR adds optional trampoline fee override parameters for outgoing Lightning payments.

The following endpoints now accept:

• trampolineFeeBaseSat
• trampolineFeeProportional
• trampolineFeeCltvExpiryDelta

Supported endpoints:

• /payinvoice
• /payoffer
• /paylnaddress
• /lnurlpay

phoenix-cli also exposes matching flags for the same commands.

When these parameters are absent, behavior is unchanged and phoenixd uses its default configured trampoline fee.

Context

This came out of investigating Ambrosia PoS using phoenixd as its Lightning backend.

In the observed failures, Ambrosia submitted outgoing payments to phoenixd, phoenixd accepted the payment attempt, sent an HTLC through its trampoline/LSP peer, and then received a downstream failure. One plausible failure mode is that the trampoline node may require a higher fee or CLTV delta than phoenixd’s default single trampoline fee tier.

lightning-kmp already supports trampolineFeesOverride on outgoing payment commands. Phoenix mobile also has a similar concept where the send flow can pass a selected TrampolineFees value. phoenixd did not expose that capability through its HTTP API or CLI.

This PR exposes that existing lower-level capability so clients can opt into a higher trampoline fee policy when appropriate, without changing phoenixd’s default fee behavior for all users.

Note: this PR depends on ACINQ/lightning-kmp#875, which exposes trampoline fee overrides in the high-level Peer.payInvoice(...) and Peer.payOffer(...) APIs.

Compatibility

This is intended to be backward-compatible:

• existing requests without the new parameters behave exactly as before
• no default trampoline fee values are changed
• no database migration is required
• payment behavior only changes when the caller explicitly provides an override

Notes

The override is intentionally per request. This avoids silently increasing fees globally and lets clients decide when a higher trampoline fee is acceptable for their own UX or retry policy.
The CLTV override is optional. If only trampolineFeeBaseSat and trampolineFeeProportional are provided, phoenixd keeps the default trampoline CLTV expiry delta.

Tests

Added tests for:

• no override when parameters are absent
• valid override construction
• default CLTV fallback
• partial override rejection
• invalid value rejection

Tested with:

./gradlew jvmTest --tests "fr.acinq.phoenixd.TrampolineFeesOverrideTestsCommon"
./gradlew jvmTest

[pm47]

This needs to be done in lightning-kmp.

[ca-ruz]

Fixed. The manual PayInvoice / PayOffer send/wait flow has been removed from phoenixd. This now uses the high-level lightning-kmp APIs added in ACINQ/lightning-kmp#875.

[ca-ruz]

Updated this PR based on the review feedback.

Main changes:

• removed the phoenixd-side manual PayInvoice / PayOffer send/wait flow
• now uses the high-level lightning-kmp payment APIs from Expose trampoline fee overrides in Peer payment APIs lightning-kmp#875
• kept phoenixd responsible only for HTTP parameter parsing and validation
• kept the API parameters unchanged for phoenixd clients
• updated the lightning-kmp dependency to 1.13.2-SNAPSHOT

Verification:

• ./gradlew jvmTest --tests "fr.acinq.phoenixd.TrampolineFeesOverrideTestsCommon"
• ./gradlew jvmTest