fix(sdk): harden order salt randomness, explicit signer resolution, and cancellation safety

[mertcano]

Description

This PR hardens the TypeScript SDK core order builder and cancellation paths against predictable order identifiers, implicit non-null assertions, and missing signer contracts[cite: 37]. It introduces cryptographic randomness for order salts, ensures fail-fast error handling for unassigned signers, and preserves zero-salt semantics[cite: 37].

Key Changes

• Cryptographic Order Salts (sdk/src/OrderBuilder.ts, sdk-python/...):
  • Replaced non-cryptographic pseudo-random number generators (Math.random() / random.randint) with cryptographically secure random bytes and rejection sampling to completely eliminate modulo bias[cite: 37].
• Explicit Signer Resolution (sdk/src/OrderBuilder.ts):
  • Replaced unsafe non-null assertions (this.signer!.address) with strict typed failure handling, ensuring MissingSignerError is thrown predictably before any property dereference[cite: 37].
• Predict-Account & Maker/Signer Semantics (sdk/tests/OrderBuilder.test.ts):
  • Added robust test coverage confirming correct maker/signer precedence and fallback behaviors when utilizing Predict accounts[cite: 37].

Validation & Testing

• TypeScript Quality Gates: Successfully passed yarn typecheck, yarn lint, and the full TypeScript regression suite (yarn test / Jest on-chain/unit tests).

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Note

Medium Risk
Changes affect how every default order gets its salt and how maker/signer are chosen before signing; incorrect resolution could break order submission, but scope is limited to buildOrder and generateOrderSalt.

Overview
Default order salts now come from ethers randomBytes with rejection sampling into [0, MAX_SALT], replacing Math.random() so salts are unpredictable and not modulo-biased.

buildOrder resolves the signing identity as data.signer → connected wallet address → predictAccount, and throws MissingSignerError when none exist instead of using this.signer!.address. Maker/signer mismatch is enforced only for non–Predict-account builders; Predict-account mode still forces maker and signer to the predict account.

Tests add coverage for missing-signer failure and Predict-account maker/signer fields.

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