refactor!: remove go-fraud dependency and related dead code

[rootulp]

Summary

Closes #4930

Remove the github.com/celestiaorg/go-fraud dependency from celestia-node. Fraud proofs are no longer functional post-shwap and there are no plans to reintroduce them. This reduces maintenance burden and allows archiving the go-fraud repository.

Breaking change

The fraud RPC namespace is removed. Node operators and clients that call fraud.Subscribe or fraud.Get will receive errors after upgrading. The Fraud field is also removed from the Go RPC client.

These endpoints have been non-functional since the shwap migration — fraud proofs were never produced or validated post-shwap, so these methods returned no useful data. No action is needed unless you have tooling that explicitly calls the fraud RPC namespace.

What was removed

• nodebuilder/fraud/ — entire package (Module interface, API, ServiceBreaker, constructors, proof unmarshaler, mocks)
• share/eds/byzantine/bad_encoding.go — BadEncodingProof (BEFP) implementation and tests
• share/eds/byzantine/pb/ — protobuf definitions for BEFP serialization
• header/headertest/fraud/ — FraudMaker test helper
• nodebuilder/tests/fraud_test.go — fraud integration test (was already skipped)
• Fraud pubsub topics — removed from gossipsub scoring
• Fraud RPC endpoint — removed from server registration and client
• github.com/celestiaorg/go-fraud — removed from go.mod

What was preserved

• share/eds/byzantine/byzantine.go — ErrByzantine type (still used by cascade getter and full availability for error classification)
• share/eds/byzantine/share_proof.go — ShareWithProof type and GetShareWithProof (still used by ErrByzantine)

Test plan

• go build ./... passes
• go test ./das/ passes
• go test ./share/eds/ passes (including renamed TestByzantineError)
• go test ./api/... passes (including TestModulesImplementFullAPI)
• scripts/check-go-mod-parity.sh passes
• No remaining references to go-fraud, nodebuilder/fraud, BadEncodingProof, or ServiceBreaker

Release notes

The following should be included in the release notes for the next release that contains this change:

Breaking: fraud RPC namespace removed

The fraud.Subscribe and fraud.Get RPC methods have been removed. These endpoints have been non-functional since the shwap migration and returned no useful data. If you have monitoring or tooling that calls the fraud RPC namespace, remove those calls before upgrading. No other action is required — this change does not affect node operation, syncing, or data availability sampling.

🤖 Generated with Claude Code

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 5 additional findings.

[walldiss]

Goodbye BEFPs

[rootulp]

@vgonkivs can this PR be included in the next celestia-node release so that we can archive go-fraud. Concretely this PR doesn't need to be included in the v0.30.x release line that we're currently pushing through testnets. But it would be nice to include it in v0.31.x and/or the celestia-node release.

Ref: celestiaorg/go-fraud#143