feat: support split L2 HTTP and WebSocket RPC

[davidtaikocha]

🚀 What’s this PR do?

Ports NethermindEth/Catalyst#988 onto the current taikoxyz/Catalyst master, with follow-up hardening from review.

• Add optional L2_WS_RPC_URL for L2 block subscriptions.
• Keep L2_RPC_URL for ordinary execution-layer JSON-RPC requests, allowing HTTP transport.
• Preserve compatibility by reusing L2_RPC_URL when it already uses ws:// or wss:// and L2_WS_RPC_URL is unset or blank.
• Validate both RPC URLs, select transport from the parsed scheme, and preserve canonical URLs through both provider and subscription connection paths.
• Route the Shasta and Realtime chain monitors through l2_ws_rpc_url; Pacaya and execution-layer providers remain on l2_rpc_url.
• Document the opt-in in .env.sample and CHANGELOG.md.

Deployment configuration must provide L2_WS_RPC_URL before switching L2_RPC_URL to HTTP.

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📎 Related issues (optional)

• Upstream implementation: feat: support split L2 HTTP and WebSocket RPC NethermindEth/Catalyst#988
• Related rollout: chore(taiko-client,taiko-client-rs): refuse shutdown ahead of imminent sequencing windows taiko-mono#22000
• Related rollout: https://github.com/taikoxyz/k8s-configs/pull/1461

🧠 Context

eth_subscribe requires WebSocket, but ordinary L2 execution requests do not. When operators configure distinct HTTP(S) and WS(S) endpoints, subscription reconnect failures no longer share the ordinary request transport. If L2_RPC_URL remains WS(S) and L2_WS_RPC_URL is unset, both paths intentionally continue to use the same endpoint for backwards compatibility.

The initial implementation ports upstream commit b8d555315e5e9a99c9d5dbad91935bf364d0a308; follow-up commits handle blank environment values, validate the ordinary L2 RPC URL, centralize parsed-scheme transport selection, preserve canonical WebSocket URLs through the direct subscription path, and add documentation and regression tests.

🧪 Validation

• cargo test --workspace --locked --verbose
• cargo test -p common config::tests -- --nocapture (15 passed)
• cargo test -p common --lib (56 passed)
• cargo clippy --workspace --all-features --exclude p2p-boot-node --locked -- -D warnings
• cargo fmt --all -- --check
• cargo sort --workspace --check
• git diff --check

typos and cargo-deny were not installed locally; the PR CI runs both. This port does not modify dependencies or Cargo.lock.

✅ Checklist

• I’ve tested this locally
• I’ve added relevant docs or comments
• I’ve updated or created tests if needed
• The branch with the feature is named feature/<name>