feat: Implement transaction builder and fee estimation module (#45)

[amankoli09]

Title:

feat: Implement Transaction Construction and Fee Estimation Module (#45)

Closes #45

📖 Description & Motivation

This PR resolves #45 by delivering a unified, safe, and heavily tested transaction-construction module for the Vero SDK.

Historically, transaction building was duplicated across services (vero-relayer-service and vero-guardian-dashboard) with divergent default configurations for fees, timeouts, and networks. This fragmentation often led to non-obvious failure states, particularly during network congestion. Crucially, the lack of upper bounds on network fee polling exposed the protocol to fund-loss risks during network fee spikes.

This PR establishes a centralized transaction module to standardize building transactions, managing data payloads, securing fee bounds, and properly configuring fee bump transactions without consuming additional sequence numbers.

🏗️ Proposed Changes

• Transaction Builder Wrapper (buildTransaction): Introduced a standardized abstraction over stellar-sdk's TransactionBuilder to enforce centralized timeout and network configurations.
• Manage Data Abstraction (buildManageData): Centralized logic for interpreting protocol-specific data keys (e.g., reputation, task, vote) and generating the exact binary XDR payloads.
• Fee Bump Integration (buildFeeBump): Added logic to construct fee bump transactions that correctly utilize the inner transaction's sequence number.
• Safe Fee Estimation (estimateFee): Introduced a bounded fee estimation mechanism that fetches live statistics from the RPC node, clamps them between configured minFee and maxFee limits, and securely fails on edge-case exceptions.
• Jest Configuration Overhaul: Enhanced the local jest.config.js to ensure proper ES Module (ESM) resolution for the underlying @stellar/stellar-sdk and its sub-dependencies (uint8array-extras, @exodus, @noble).

📂 Files Changed

File Path	Type	Description
src/transaction/builder.ts	New	Exposes buildTransaction, buildManageData, and buildFeeBump using type-safe generics over stellar-sdk.
src/transaction/fees.ts	New	Enforces the bounded estimateFee routine, resolving network-spike vulnerabilities.
src/transaction/index.ts	New	Module entry point exporting all builder and fee abstractions.
src/index.ts	Modified	Updated the global SDK export to expose the new transaction module.
src/__tests__/transaction.test.ts	New	Comprehensive test suite enforcing logic boundaries, XDR parsing checks, and RPC exception handling.
package.json / package-lock.json	Modified	Installed @stellar/stellar-sdk as a direct dependency for the SDK wrapper functions.
jest.config.js	Modified	Updated transformIgnorePatterns regex to parse underlying ESM imports gracefully during test runtime.

🧠 Design Decisions & Architectural Notes

1. Type Safety over any: In buildManageData, unknown was utilized with robust type-casting (e.g., as string | Buffer | null) to ensure TypeScript correctly infers logic flows without resorting to unsafe any assertions, preserving the strict linting environment.
2. XDR De-serialization in Testing: Instead of interacting directly with raw JS-XDR buffers in tests, Operation.fromXdrObject was used. This ensures that the generated ManageData payload strictly adheres to what the actual Stellar network will receive.
3. Graceful Failures (VeroError): If the fee bounds provided via configuration are invalid (i.e. minFee > maxFee), the estimation process deliberately halts and emits a strongly-typed VeroErrorCode.Unknown, allowing upstream services to catch and gracefully exit.

🧪 Testing & Validation

All configurations have been strictly verified. The test suite simulates actual payload generation and evaluates the resulting XDR signatures.

Test Results:

> @vero-protocol/sdk@0.1.0 test
> jest

PASS src/account/__tests__/data.test.ts
PASS src/account/__tests__/loader.test.ts
PASS src/__tests__/errors.test.ts
PASS src/__tests__/nonce.concurrency.test.ts
PASS src/__tests__/transaction.test.ts
PASS src/account/__tests__/index.test.ts
PASS src/account/__tests__/balances.test.ts
PASS src/__tests__/network.test.ts
PASS src/__tests__/rpc.test.ts
PASS src/__tests__/types.test.ts

Test Suites: 10 passed, 10 total
Tests:       132 passed, 132 total
Snapshots:   0 total
Time:        2.956 s
Ran all test suites.

[N-thnI]

please ensure all checks pass

[amankoli09]

@N-thnI will make it pass