test(#319, #320): add comprehensive test coverage for api.ts and contract.ts

[Uchechukwu-Ekezie]

Fixes Parashield-Protocol#319 Parashield-Protocol#320

Summary

Implements comprehensive test coverage for two critical library modules:

Issue Parashield-Protocol#319 - api.ts tests:

• Tests unwrap() function throws ApiError on {success:false} response even with HTTP 200 status (critical issue API responses with success: false on HTTP 200 are treated as successful Parashield-Protocol/parashield-frontend#198)
• Tests withRetry() behavior: retries on 5xx errors with exponential backoff, throws immediately on 4xx errors
• Tests auth interceptor correctly adds Bearer token to requests when session exists
• Tests auth error handler invoked on 401 responses
• Tests endpoint error handling for 404 responses in fetchClaim() and fetchOracleReading()

Issue Parashield-Protocol#320 - contract.ts tests:

• Tests buildBuyPolicyTx() encodes oracle key as {type: 'string'} not {type: 'symbol'} (regression test for issue Oracle key encoded as Soroban symbol — invalid charset breaks buy_policy for most products Parashield-Protocol/parashield-frontend#194)
• Tests simulateContractCall() throws ContractError with sanitized user-facing message and raw diagnostic in .details
• Tests transaction building, signing, and submission flows for policy purchase and claim operations
• Tests poll-for-confirmation timeout handling and FAILED transaction detection
• Tests RPC singleton caching for connection pool efficiency (issue contract.ts: getRpc() creates a new StellarRpc.Server instance on every call instead of caching it Parashield-Protocol/parashield-frontend#129)
• Tests BuyPolicyResult includes signedXdr for backend verification (issue Buy-policy flow never notifies the backend — api.ts's buyPolicy() is dead, disconnected code Parashield-Protocol/parashield-frontend#195)

Test coverage

• Added src/lib/__tests__/api.test.ts - 292 lines covering api module
• Added src/lib/__tests__/contract.test.ts - 260 lines covering contract module
• Total: 552 lines of test code

Related issues

• API responses with success: false on HTTP 200 are treated as successful Parashield-Protocol/parashield-frontend#198 - HTTP 200 with {success:false} should throw error
• Oracle key encoded as Soroban symbol — invalid charset breaks buy_policy for most products Parashield-Protocol/parashield-frontend#194 - Oracle key encoding regression (symbol → string)
• Buy-policy flow never notifies the backend — api.ts's buyPolicy() is dead, disconnected code Parashield-Protocol/parashield-frontend#195 - Pass signed XDR to backend
• contract.ts: getRpc() creates a new StellarRpc.Server instance on every call instead of caching it Parashield-Protocol/parashield-frontend#129 - RPC connection pool efficiency

Testing

These tests verify critical error handling paths and transaction encoding that impact production behavior. No breaking changes to existing code.