fix(consensus): prev hash byron genesis support

[wolf31o2]

This replaces #1623 and enhances #1628's implementation

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Summary by cubic

Fixes previous-hash validation and header handling for Byron genesis transitions. Non-genesis blocks now require a previous header hash; genesis transitions may omit it, with consistent mismatch errors.

• Bug Fixes
  • In consensus/byron, detect genesis via PrevBlockNumber; require PrevHeaderHash only for non-genesis; set prev header fields only when prevHeader != nil in ValidateByronBlockHeader.
  • In consensus, enforce the same rule via BlockNumber and keep a single error message.
  • Tests cover matching, missing, mismatched, and genesis transition cases with exact message checks.

^{Written for commit bc42598. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes

  • Added validation to ensure the previous header hash is required for non-genesis blocks, preventing incomplete block validation.

• Tests

  • Enhanced test coverage with additional validation scenarios, including explicit handling of missing previous header hash cases and improved error message assertions.

[coderabbitai]

📝 Walkthrough

Walkthrough

The pull request introduces validation logic to require the presence of PrevHeaderHash for non-genesis blocks across the consensus validation code. Changes are made in both the Byron-specific and generic validate modules, with corresponding test cases added to verify the new validation behavior. The validation now returns an explicit error when non-genesis blocks lack a PrevHeaderHash, while genesis blocks remain unaffected. Code formatting adjustments collapse multi-line statements into single-line format without altering functionality.

Possibly related issues

• PrevHash validation silently skipped on empty input #1576: Directly addresses the same validation requirement by modifying validatePrevHash to enforce PrevHeaderHash presence for non-genesis blocks and adding corresponding test coverage.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: adding validation safeguards for the previous header hash in Byron consensus, with specific handling for genesis blocks.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/prev-header-improvements

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[cubic-dev-ai]

No issues found across 4 files

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@consensus/byron/validate.go`:
- Around line 231-234: The exported Byron header wrappers
(ValidateByronBlockHeader / ValidateByronMainBlockHeader /
ValidateByronEBBHeader) still assume a non-nil prevHeader and unconditionally
call prevHeader.SlotNumber(), prevHeader.BlockNumber(), and prevHeader.Hash(),
preventing genesis/no-prev validation; update these wrappers to accept a nil
prevHeader when input represents genesis (e.g., PrevBlockNumber == 0 or empty
PrevHeaderHash) and short-circuit any reads from prevHeader (skip
slot/number/hash comparisons) so the lower-level validatePrevHash path can
handle the no-prev case; ensure ValidateHeaderInput can carry a nil PrevHeader
and that callers pass nil instead of a dummy header so validatePrevHash and
related checks operate correctly.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: f67780cb-1758-422e-b898-75e49b76650f

📥 Commits

Reviewing files that changed from the base of the PR and between b410b00 and b5c12d1.

📒 Files selected for processing (4)

• consensus/byron/validate.go
• consensus/byron/validate_test.go
• consensus/validate.go
• consensus/validate_test.go