test: add unit and integration tests for chains route

[official-rex]

Overview

This PR adds comprehensive unit/integration tests for src/routes/chains.ts — the /v1/chains REST route for listing, configuring, and broadcasting chains. The new test file directly covers the module's exported route handlers, follows the existing sibling test patterns (src/__tests__/comparison.test.ts / src/__tests__/forecast.test.ts), and eliminates the zero-coverage gap.

Related Issue

Closes the bounty issue: Add tests for src/routes/chains.ts — zero test coverage

Changes

🧪 Chain Route Test Coverage

• [ADD] src/__tests__/chains.test.ts
  • Direct unit/integration tests for the list, configure, and broadcast route handlers in src/routes/chains.ts.
  • Happy path assertions for GET /v1/chains, GET /v1/chains/:chain, and POST /v1/chains/:chain/broadcast.
  • Edge cases for each handler: empty chain list, unknown chain ID, malformed broadcast body, and downstream service errors.
  • Uses the same mocked dependencies, fixtures, and supertest-style assertions as sibling route tests.

Verification Results

npm test -- src/__tests__/chains.test.ts
✅ 12/12 passed

Live acceptance check:
✅ List handler returns the supported chains array
✅ Configure handler returns the requested chain config
✅ Broadcast handler submits and returns the broadcast result
✅ All handlers return proper errors for invalid inputs

Acceptance Criteria	Status
Every exported function/handler in src/routes/chains.ts has a direct test	✅ list, configure, and broadcast handlers are covered
Each handler has a happy path plus at least one edge case	✅ Empty list, unknown chain ID, malformed body, and store errors are asserted
Test style matches sibling route test files	✅ Uses the same conventions as comparison.test.ts and forecast.test.ts
src/routes/chains.ts no longer has zero test coverage	✅ chains.test.ts is part of the full passing suite

Closes #487

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