[feature]: Support MPP record (alias Payment_Address) for more RPC calls

[kwsantiago]

Relates to #9952

Core Tasks:

• QueryRoutes should support including an MPP record (payment_addr)
• SendToRoute should enforce an MPP record
• BuildRoute should enforce payment_secret
• Basic Tests
• Comprehensive Performance Testing
• Production Hardening, Documentation & Monitoring

Extras:

• Allow AMP Record in buildroute
• Allow Setting AMP record for QueryRoutes
• Allow SendToRoute with an AMP record

[gemini-code-assist]

Summary of Changes

Hello @kwsantiago, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances LND's support for Multi-Path Payments (MPP) by introducing a robust validation framework. The changes ensure that key routing RPCs like QueryRoutes, SendToRoute, and BuildRoute can properly handle and enforce MPP records (payment addresses), aligning with best practices for payment reliability. This new system is highly configurable, allowing operators to gradually transition to stricter enforcement while providing clear observability into the validation process.

Highlights

• MPP Validation Framework: Introduced a new MPPValidator component (lnrpc/routerrpc/mpp_validator.go) to centralize and manage Multi-Path Payment (MPP) record validation across various routing RPCs.
• Enhanced RPC Support: The QueryRoutes RPC now accepts an optional payment_addr (MPP record) to be included in the final hop of the returned route. Additionally, SendToRoute and BuildRoute RPCs now enforce the presence and correctness of MPP records based on configurable modes.
• Configurable Enforcement: Implemented flexible enforcement modes (legacy, warn, enforce) for MPP validation, allowing global settings, per-RPC overrides, an emergency bypass, and a grace period for gradual adoption. These settings are exposed via a new MPPConfig in lnrpc/routerrpc/routing_config.go.
• Observability: Added new Prometheus metrics to track MPP validation attempts, failures, rejections, warnings, and mode changes, improving monitoring capabilities for payment routing.

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[gemini-code-assist]

Code Review

This pull request introduces support for MPP records (alias Payment_Address) for more RPC calls, including QueryRoutes, SendToRoute, and BuildRoute. It also includes basic tests and configuration options for MPP validation.

[gemini-code-assist]

The function comment should be a complete sentence and start with the function name.¹

// BuildRoute builds a route from a list of hop addresses.

Style Guide References

Footnotes

1. Function comments must begin with the function name. (link) ↩