feat(statics): give Sepolia testnet its own underlying asset (sepETH)

[rithwik-bitgo]

Summary

• Onboards Sepolia (chain ID 11155111) as a testnet-only network for Ethereum, following the existing gteth/hteth pattern (CoinFamily.ETH, no mainnet counterpart since Sepolia is Ethereum's own test network).
• Gives the coin its own UnderlyingAsset.SEPETH instead of reusing UnderlyingAsset.ETH, so it's distinguishable as its own asset while still sharing the eth family/signing implementation.
• Coin code: sepeth ("Sepolia Testnet Ethereum"). OFC entry: ofctsepeth (kept ofct-prefixed to satisfy the OFC testnet naming-convention test).

Notes

• batcherContractAddress / forwarderFactoryAddress / forwarderImplementationAddress are intentionally omitted (optional fields on EthereumNetwork) since BitGo's forwarder/batcher contracts haven't been deployed on Sepolia yet — to be added once contract deployment is complete.
• Considered making SEPETH a fully standalone CoinFamily, but reverted: getEthLikeTokens (AMS token config tooling) requires every family with an EVM-feature coin to have a canonical mainnet coin named exactly like the family, which a testnet-only chain can't satisfy without adding a fake mainnet entry. Keeping family = CoinFamily.ETH avoids that while still giving Sepolia its own asset identity.
• No changes needed to networkFeatureMapForTokens.ts (already covers the eth family) or environments.ts (its explorer config is a single shared endpoint per environment, not per-testnet).

CI fix (unrelated to Sepolia)

• unit-test was failing in @bitgo/sdk-coin-stx (Invalid character in hello) after bn.js was bumped to 5.2.3 (WCN-1224 / chore: update bn.js to version 5.2.3 in package.json and yarn.lock #9349).
• @stacks/transactions signWithKey passes the message to elliptic as hex. Test fixture message2 = 'hello' is not valid hex; it only passed when elliptic resolved its nested lenient bn.js@4.
• Fix: test-only change in sdk-coin-stx fixtures — message2 → valid hex deadbeef, regenerated expectedSignature2. No production signing behavior change.

TICKET: CECHO-1721

Test plan

• yarn workspace @bitgo/statics run unit-test — all 33k+ tests passing
• tsc --noEmit typecheck clean for modules/statics
• yarn workspace @bitgo/sdk-coin-stx run unit-test — sign/verify fixtures pass under bn.js@5
• Contract deployment + forwarder address wiring (follow-up)

[linear-code]

CECHO-1721

[manojkumar138]

CI unblock: stx sign/verify fixture

unit-test failures were not from the sepeth statics change. They came from @bitgo/sdk-coin-stx:

• Root cause: after WCN-1224 / #9349 (bn.js@5.2.3), CI resolves elliptic against strict bn.js@5.
• @stacks/transactions signWithKey treats the message as hex → fixture message2 = 'hello' throws Invalid character in hello.
• Fix (test-only): message2 → deadbeef, regenerated expectedSignature2 in modules/sdk-coin-stx/test/unit/resources.ts.

Commit: a4d99fcdf6