test: add property-based tests for XDR transaction builder

[jaymoneymanxl]

Closes #785

Problem

The TransactionBuilder and related XDR serialisation modules lacked property-based (fuzz) testing. Unit tests cover specific known cases, but they don't explore the vast space of valid and invalid inputs that real-world usage can produce, such as:

• Edge-case amounts (0, 1, u64::MAX, extreme decimals).
• Malformed memo payloads (oversized text, wrong hash length, invalid enum discriminants).
• Time bounds with maxTime < minTime.
• Mixed operation sets with varying types and boundaries.

Without property-based tests, regressions in XDR round-trip encoding/decoding or input validation can slip through.

Changes

src/lib/__tests__/transactionBuilder.property.test.ts (new)

Install fast-check as a devDependency and add the following property tests, each running 50–200 random iterations:

1. XDR Round-Trips (xdrRoundtrip)

• Generates a random Transaction with 1–10 operations, each of type payment, createAccount, or changeTrust.
• Encodes the transaction to XDR, then decodes it back.
• Property: The decoded fields (source account, operations, memo, time bounds, fee) are deeply equal to the original.

2. Amount Boundary Rejection (amountBoundaries)

• Generates invalid amount inputs: 0, negative numbers, Infinity, -Infinity, NaN, extreme floats (> u64::MAX), non-integer decimals.
• Property: TransactionBuilder.isValidAmount() returns false for every invalid input.
• Also generates valid amounts (1 … u64::MAX) and verifies they return true.

3. Memo Boundary Handling (memoBoundaries)

• Generates valid text memos (1–28 chars, printable ASCII), ID memos (u64 range), hash memos (exactly 32 bytes), and return hash memos.
• Property: Each valid memo round-trips through Memo.toXDR() / Memo.fromXDR() without error.
• Generates invalid memos: text > 28 bytes, hash with wrong length (≠ 32), empty text, null/undefined.
• Property: Each invalid input throws or returns a validation error.

4. Time Bounds Round-Trip (timeBoundsRoundtrip)

• Generates random minTime/maxTime pairs where minTime <= maxTime.
• Property: Serialising and deserialising through XDR preserves both values exactly.
• Also verifies that minTime > maxTime pairs are rejected.

5. Operation Set Boundaries (operationSetBoundaries)

• Generates transactions with varying operation counts (0, 1, 5, 10, 20, 50) and mixed types.
• Property: Transactions with 0 operations raise a validation error.
• Property: Transactions with 1–10 valid operations are accepted and round-trip correctly.
• Property: Each operation in the decoded set retains its original type and parameters.

package.json

• Added fast-check: ^3.x devDependency.

Verification

• Run npx vitest run src/lib/__tests__/transactionBuilder.property.test.ts → all 5 property suites pass.
• Each suite runs 50–200 random iterations, covering a combinatorial space far larger than hand-written unit tests.
• Add a deliberate regression (e.g., remove XDR encoding) → at least one property test fails, preventing the regression.

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