feat: add estimateFee() for stream operations

[GazzyLee]

Summary

Before a user clicks Create Stream, the UI needs to display the estimated network fee. This PR adds estimateFee(operation: StreamOperation) to StreamsModule, which runs a Soroban simulation and returns an exact fee breakdown based on the transaction's CPU/RAM resource usage and the current base fee.

What's included

• src/types/index.ts

  • New StreamOperation discriminated union covering all 7 stream operations: create, withdraw, cancel, pause, resume, topUp, clawback
  • New FeeEstimate interface: { totalFee, resourceFee, baseFee, instructions } (all in stroops)

• src/streams.ts

  • New async estimateFee(operation: StreamOperation): Promise<FeeEstimate> method on StreamsModule
  • Builds the contract-call Transaction for the requested operation using existing helpers (buildContractCallTx, getTokenDecimals, toStroops, calculateRate, _resolveAddr)
  • Simulates the transaction via Soroban RPC and extracts:
    • Resource fee from simResult.minResourceFee (CPU/RAM cost)
    • CPU instructions from simResult.cost.cpuInsns
    • Base fee from the SDK's BASE_FEE constant
  • Handles RPC rate limits via RateLimitError.fromRpcError() and simulation errors via SorobanRpc.Api.isSimulationError() — no as any property guessing

Example

const client = new ConduitClient({ network: 'mainnet', keypair });

const estimate = await client.streams.estimateFee({
  type: 'create',
  token: 'USDC',
  sender: keypair.publicKey(),
  recipient: 'G...',
  depositAmount: '1000',
  durationSeconds: 2592000,
});

console.log(estimate);
// { totalFee: 3149, resourceFee: 3049, baseFee: 100, instructions: 2048576 }

### Validation
tsc --noEmit — passes
npm run lint — passes
All 500 existing tests pass (no test files modified)

# Closes #353