test: add API route tests and fix test suite (#65)

[EmmanuelNiyonshuti]

This PR fixes the existing test setup and adds missing API route tests. Addresses #65

Small fixes on the existing test setup, shared fixtures were scattered across tests/fixtures/ instead of conftest.py so, I moved them there. Renamed test_jwt_token to jwt_token since the test_ prefix is for test functions. Added missing fixtures for response shapes that the new tests needed.

Added two new unit tests for get_thi in TestOpenWeatherMap , one for the default path and one for ocsm=True.

Added tests/api/ with three new test files:

• test_data_routes.py all routes in src/api/api.py, testing 200, 500, and 403 per route
• test_locations.py integration tests hitting the in-memory mock DB directly. Two routes that depend on MongoDB $near geospatial queries are skipped since mongomock doesn't support them
• test_history.py both history routes. Only the cache miss path is testable for the same $near reason. Skipped tests are left in with comments for whoever picks them up later.