fix(models): coerce hex-string quantities in from_dict

[devorun]

TempoTransaction.from_dict accepts camelCase keys (chainId, maxFeePerGas, gas, value, ...), which is the Ethereum JSON-RPC convention — where quantities are hex strings (e.g. "0x64"). But it passed those values through unconverted, so parsing a standard JSON-RPC transaction dict raised TypeError: '<' not supported between instances of 'str' and 'int' during validate().

TempoTransaction.from_dict({"chainId": "0xa5bd", "gas": "0x186a0", "value": "0x64", ...})  # TypeError

This coerces quantity fields (chain_id, max_fee_per_gas, max_priority_fee_per_gas, gas_limit, nonce, nonce_key, valid_before, valid_after, and per-call value) via a small as_int helper that accepts hex strings, decimal strings, and ints. Integer and snake_case inputs are unaffected. Adds tests for both hex-string and int inputs.