Feature/session signer

[pradeep-selvaraju]

Note

Medium Risk
Changes order-signing behavior for SignatureTypeV2.POLY_1271 by introducing an alternate signing path and a new signature encoding, which could break compatibility if the on-chain/off-chain verifier expects a different format.

Overview
Adds optional sessionSigner support that can be passed via ClobClientOptions and is threaded through OrderBuilder/order creation helpers into ExchangeOrderBuilderV2.

For v2 POLY_1271 orders, signing can now use the provided sessionSigner to produce a wrapped/nested signature payload (tagged with SESSION_SIGNER_MAGIC); if absent, it falls back to requiring a viem WalletClient local account, and error messaging is updated accordingly.

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