fix(anvil): fix multi-fork-url bootstrap race, stale docs, and RoundRobinService guard

[zerosnacks]

Summary

Follow-up to #14280. Fixes medium and minor issues identified during review.

Changes

Bootstrap race condition fix

setup_fork_db_config() now uses the primary URL only for bootstrap/discovery calls (get_chain_id, find_latest_fork_block, get_block, get_gas_price). The round-robin provider is built after the fork block is pinned, preventing intermittent startup failures when endpoints are at slightly different chain tips.

is_local detection fix

build_fallback() now uses normalized/parsed URLs for guess_local_url(), consistent with how build() computes is_local. Previously it ran guess_local_url on raw input strings, which could misclassify localhost:8545, bare socket addresses, or IPC paths.

RoundRobinService::new() guard

Added a panic guard for empty transport lists to prevent a % 0 panic in call().

Stale docs/comments

Fixed all references to Alloy FallbackService and health-scored routing:

• build_fallback() docstring → now describes round-robin + retry
• CLI --fork-url help text → removed "best-performing endpoint" claim
• NodeConfig.fork_urls comment → "round-robin load balancing with retry-based failover"
• ClientForkConfig.fork_urls comment → same
• setup_fork_db_config debug log → "round-robin" instead of "fallback"

Testing

cargo test -p foundry-config -- endpoints
cargo test -p anvil --lib cmd::tests
cargo test -p anvil --test it -- test_anvil_set_rpc_url_syncs_fork_config test_anvil_reset_with_url_updates_fork_urls