Use proposer lookahead for data column verification (#16202)

Replace the proposer indices cache usage in data column sidecar
verification with direct state lookahead access. Since data column
sidecars require the Fulu fork, the state always has a ProposerLookahead
field that provides O(1) proposer index lookups for current and next
epoch.

This simplifies SidecarProposerExpected() by removing:
- Checkpoint-based proposer cache lookup
- Singleflight wrapper (not needed for O(1) access)
- Target root computation for cache keys

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

testing/util/deneb.go

@@ -44,6 +44,13 @@ func WithProposerSigning(idx primitives.ValidatorIndex, sk bls.SecretKey, valRoo
 	}
 }
 
+// WithProposer sets the proposer index for the generated block without signing.
+func WithProposer(idx primitives.ValidatorIndex) DenebBlockGeneratorOption {
+	return func(g *denebBlockGenerator) {
+		g.proposer = idx
+	}
+}
+
 func WithPayloadSetter(p *enginev1.ExecutionPayloadDeneb) DenebBlockGeneratorOption {
 	return func(g *denebBlockGenerator) {
 		g.payload = p