fix(types): validate nil SignedHeader in LightClientAttackEvidence (#2923)

Closes
[PROTOCO-1518](https://linear.app/celestia/issue/PROTOCO-1518/chain-halt-via-nil-signedheader-in-lightclientattackevidence).
Backports
[cometbft/cometbft#5757](cometbft/cometbft#5757)
([`336b47c`](cometbft/cometbft@336b47c))
to celestia-core `main`.

## Test plan
- [x] `go test ./types/...` passes locally
- [ ] CI green

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
(cherry picked from commit 15e142c)

Author: 2 people

.changelog/unreleased/bug-fixes/lcae-nil-signed-header.md

@@ -0,0 +1,3 @@
+- `[types]` Validate that `LightClientAttackEvidence.ConflictingBlock.SignedHeader`
+  is non-nil in `ValidateBasic` to prevent a nil pointer panic during block
+  deserialization. Backported from cometbft/cometbft#5757.

types/evidence.go

@@ -358,6 +358,9 @@ func (l *LightClientAttackEvidence) ValidateBasic() error {
 	}
 
 	// this check needs to be done before we can run validate basic
+	if l.ConflictingBlock.SignedHeader == nil {
+		return errors.New("conflicting block missing signed header")
+	}
 	if l.ConflictingBlock.Header == nil {
 		return errors.New("conflicting block missing header")
 	}

types/evidence_test.go

@@ -195,6 +195,7 @@ func TestLightClientAttackEvidenceValidation(t *testing.T) {
 		}, false},
 		{"Nil conflicting header", func(ev *LightClientAttackEvidence) { ev.ConflictingBlock.Header = nil }, true},
 		{"Nil conflicting blocl", func(ev *LightClientAttackEvidence) { ev.ConflictingBlock = nil }, true},
+		{"Nil signed header", func(ev *LightClientAttackEvidence) { ev.ConflictingBlock.SignedHeader = nil }, true},
 		{"Nil validator set", func(ev *LightClientAttackEvidence) {
 			ev.ConflictingBlock.ValidatorSet = &ValidatorSet{}
 		}, true},