Extract and expand integration tests for swap tools

[ongrid]

Background

The estimate_same_chain_swap tests currently live inside tests/server.test.ts alongside general server tests (initialization, tool listing, search_tokens, get_supported_chains). They already use mocked fetch with JSON fixtures from tests/fixtures/live-api/. However, create_tx (cross-chain swaps) has no test coverage at all.

Goals

1. Add create_tx integration tests — cover the cross-chain swap tool with mocked fixtures, similar to how estimate_same_chain_swap is tested today:
   - ETH→USDC cross-chain (e.g. Ethereum → Arbitrum)
   - Stablecoin cross-chain (e.g. USDC on Polygon → USDC on BNB Chain)
   - Solana as source or destination chain
   - Error case: invalid/missing required params
   - Error case: unsupported chain pair
2. Extract tool-specific tests to separate files:
   - tests/estimate-same-chain-swap.test.ts — all estimate_same_chain_swap tests (currently in server.test.ts)
   - tests/create-tx.test.ts — new create_tx tests
   - tests/server.test.ts — keep only general/clean server tests (init, tool listing, get_instructions, search_tokens, get_supported_chains)
3. Re-organize fixtures for clarity:
   - tests/fixtures/live-api/estimation/ — same-chain estimation fixtures
   - tests/fixtures/live-api/create-tx/ — cross-chain create-tx fixtures
4. Extract shared setupClient helper to a reusable module (e.g. tests/setup-client.ts) to avoid duplication across test files.

Acceptance criteria

• create_tx tool has integration tests with mocked fetch and fixture files captured from live API
• estimate_same_chain_swap tests moved out of server.test.ts into their own file
• Fixture files reorganized into subdirectories by tool
• setupClient extracted and shared across test files
• All existing tests continue to pass
• npm test runs without network calls (except tests explicitly opted in via env var)