feat(statics): add sepolia testnet for ETH

[rithwik-bitgo]

Summary

• Onboards Sepolia (chain ID 11155111) as a testnet-only network under the existing CoinFamily.ETH, following the gteth/hteth pattern. No mainnet counterpart is added since Sepolia is Ethereum's own test network.
• Adds coin sepeth ("Sepolia Testnet Ethereum") in allCoinsAndTokens.ts with the same feature set as hteth/gteth (TSS, MPCv2, multisig cold, custody, EIP-1559, bulk transaction features).
• Adds OFC entry ofctsepeth in ofcCoins.ts (kept the ofct-prefixed name to satisfy the existing OFC testnet naming-convention test).
• Updates expectedColdFeatures.ts test fixture to include sepeth in the "both" (cold + MPC) list.

Notes

• batcherContractAddress / forwarderFactoryAddress / forwarderImplementationAddress are intentionally omitted (these are optional fields on EthereumNetwork) since BitGo's forwarder/batcher contracts have not yet been deployed on Sepolia. These will be added once contract deployment for Sepolia is complete.
• No changes needed to base.ts (reuses existing ETH family/asset), networkFeatureMapForTokens.ts (already has an eth entry), or environments.ts (its explorer config is a single shared endpoint per environment, not per-testnet, so left untouched to keep this change testnet-only).

TICKET: CECHO-1721

Test plan

• yarn workspace @bitgo/statics run unit-test — all 32k+ tests passing
• tsc --noEmit typecheck clean for modules/statics
• Contract deployment + forwarder address wiring (follow-up)

🤖 Generated with Claude Code

[linear-code]

CECHO-1721