refactor(node-config): remove reth deps, add FromEnv to RPC configs

[prestwich]

[Claude Code]

Closes ENG-2078

Summary

• Remove reth, reth-chainspec, trevm, signet-evm dependencies from signet-node-config
• Remove dead fields (static_path, http_port, ws_port, ipc_endpoint) and methods (chain_spec(), spec_id(), static file accessors, port/ipc accessors)
• Add ServeConfigEnv with FromEnv derive for env-based RPC transport configuration
• Add StorageRpcConfigEnv with FromEnv derive for env-based RPC behavior configuration
• Add HostRpcConfig to signet-host-rpc with FromEnv derive for host RPC notifier configuration (provider URL, buffer capacity, backfill batch size)
• Fix signet-node-tests to work with slimmed SignetNodeConfig

Context

Part of the host context adapter refactor (ENG-2056). This ensures both host-reth and host-rpc modes have a clean, unified configuration surface. signet-node-config is now reth-free — cargo tree -p signet-node-config | grep reth returns nothing.

PR stack

Stacks on #112 (blobber abstraction)

Test plan

• cargo clippy --workspace --all-features --all-targets — clean
• cargo clippy -p signet-node-config --no-default-features --all-targets — clean
• cargo +nightly fmt — clean
• cargo t -p signet-node-config — 2 passed
• cargo t -p signet-rpc — 6 passed
• cargo t -p signet-host-rpc — 1 passed (1 ignored)
• cargo tree -p signet-node-config --depth 1 | grep reth — no output (zero reth deps)

Review Fixes

• Make ServeConfigEnv → ServeConfig conversion fallible (TryFrom) so malformed SIGNET_HTTP_ADDR/SIGNET_WS_ADDR returns an error instead of silently falling back to 0.0.0.0
• Replace ignore doctest on HostRpcConfig with compilable no_run example
• Add rustdoc examples to StorageRpcConfigEnv and ServeConfigEnv

Reviewed and updated by Claude Code.

🤖 Generated with Claude Code

[Fraser999]

Should these be non-optional in StorageRpcConfig now that we're setting defaults for the None cases?

[prestwich]

so there's a bit of an intent mismatch that we need to address here. some of these have "uncapped" behavior that we probably want to specify via a sentinel 0

setting a default value for the None case means these should be optional in the derive

[prestwich]

will fix in a second

[prestwich]

[Claude Code]

Good catch — the Option wrapping is intentional here because None has semantic meaning in the gas oracle: default_gas_price: None returns zero when no recent txs exist, ignore_price: None includes all tips (no floor), and max_price: None removes the cap.

The problem was that the From<StorageRpcConfigEnv> impl used .or(defaults.*), which meant the env path could never actually produce None — the defaults always filled in Some(...).

Fixed by adding a nonzero helper that maps 0 → None. The conversion now uses .map_or(default, nonzero), so:

• Env var unset → uses the default (Some(1 Gwei), Some(2 wei), Some(500 Gwei))
• Env var set to 0 → disables the guardrail (None)
• Env var set to any other value → uses that value

Also added [default: ...] annotations to all FromEnv desc strings across StorageRpcConfigEnv, ServeConfigEnv, and HostRpcConfig so the behavior when unset is always documented.

[Fraser999]

LGTM