CHAINS-6848 Include testnet 4 support

[rahul-aravind-opti]

Cherry-picking this commit - btcsuite#2339.

Used the correct merge-parent: git cherry-pick -m 1 4f4ea81776d6cecd8aa86851064e91feab649221
Resolved conflicts in:
.gitignore: kept upstream additions (.DS_Store, .aider*) and retained .idea once.
go.mod: updated to upstream deps (golang.org/x/crypto v0.22.0, golang.org/x/sys v0.19.0, pgregory.net/rapid v1.2.0).
wire/protocol.go: restored TestNet4 and SigNet constants and added to bnStrings.
wire/protocol_test.go: restored TestNet4 and SigNet in the stringer test list.
Continued the cherry-pick and ran:
go mod tidy
go test ./... -run='^$' (compile-only across all packages)