test(api): add API endpoint response conformance test vectors

[tcoratger]

Summary

• New ApiEndpointTest fixture that generates JSON test vectors for the 4 deterministic API endpoints
• Given genesis parameters (validator count + genesis time), computes the exact expected HTTP response
• Client teams build the same genesis store, query their endpoint, and compare against the fixture

Endpoints covered

Endpoint	Content-Type	What the vector contains
GET /lean/v0/health	application/json	Fixed status + service name
GET /lean/v0/checkpoints/justified	application/json	Slot + root from latest justified checkpoint
GET /lean/v0/states/finalized	application/octet-stream	Hex-encoded SSZ bytes of finalized state
GET /lean/v0/fork_choice	application/json	Full tree: nodes with weights, head, checkpoints, validator count

Vectors (6 total)

Test	Genesis	What it validates
health	4 validators	Static payload independent of state
justified_checkpoint_4v	4 validators	Slot 0 + genesis root
justified_checkpoint_12v	12 validators	Different root (different state root)
finalized_state_4v	4 validators	Full SSZ encoding of genesis state
fork_choice_4v	4 validators	Single node, zero weights
fork_choice_12v	12 validators	Same shape, higher validator count

Test plan

• uvx tox -e all-checks passes
• uv run fill --fork=devnet --clean -n auto -- tests/consensus/devnet/api/ generates all 6 fixtures
• Verified health returns {"status": "healthy", "service": "lean-rpc-api"}
• Verified justified root differs between 4v and 12v configurations
• Verified fork choice tree has single genesis node with weight 0

🤖 Generated with Claude Code