lnwire: validate MuSig2 nonce points on wire decode

[erickcestari]

Add point-on-curve validation for MuSig2 public nonces at the TLV decode layer. A MuSig2 nonce is 66 bytes (two 33-byte compressed secp256k1 public keys). Previously, nonce bytes were accepted without validation, with invalid points only failing later during MuSig2 session creation deep in the signing flow. Now, malformed nonces from a peer are rejected immediately at decode time with clear errors.

This hardens all nonce-carrying messages (taproot): ClosingComplete, ClosingSig, Shutdown, CommitSig, and others.

[gemini-code-assist]

Summary of Changes

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This pull request significantly enhances the robustness of the lnwire package by introducing immediate validation for MuSig2 nonce points during message decoding. By catching malformed nonces at the earliest possible stage, the system becomes more resilient to invalid or malicious input, improving overall security and stability for all messages that carry MuSig2 nonces.

Highlights

• MuSig2 Nonce Validation: Implemented point-on-curve validation for MuSig2 public nonces directly at the TLV decode layer. This ensures that incoming MuSig2 nonces are cryptographically valid secp256k1 points.
• Early Error Detection: Malformed nonces from peers will now be rejected immediately during message decoding, preventing invalid data from propagating deeper into the signing flow and causing later failures.
• Hardened Messages: This validation hardens all nonce-carrying messages, such as ClosingComplete, ClosingSig, Shutdown, and CommitSig, by ensuring the integrity of their MuSig2 nonces.
• Improved Test Utilities: Updated test utilities to generate cryptographically valid MuSig2 nonces, ensuring more robust testing of nonce-related logic.

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[Roasbeef]

LGTM 🧞‍♂️