chore: Add integration and unit tests for Python and .NET SDKs

[joakimia]

Summary

Expands the Python SDK with missing endpoint wrappers, introduces a full integration- and unit-test suite covering all Python endpoints, and adds test fake infrastructure for the .NET WhenHandlingTransientErrors test suite.

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Python SDK

New endpoint wrappers

The following endpoints existed in the generated API client but were not exposed through the SDK wrapper or HeimdallApiClient. They are now fully implemented:

grid_insights.py / client.py

Method	Type	Description
get_currents	Historical	Current measurements for a line over a time range
get_conductor_temperatures	Historical	Conductor temperature measurements for a line
get_icing	Historical	Icing measurements for a line over a time range
get_sag_and_clearance	Historical	Sag and clearance measurements for a line
get_apparent_power	Historical	Apparent power measurements for a line
get_latest_apparent_power	Latest	Most recent apparent power measurement
get_icing_forecast	Forecast	Latest icing forecast for a line

capacity_monitoring.py / client.py

Method	Type	Description
get_heimdall_dlrs	Historical	Heimdall DLR values for a line over a time range
get_heimdall_aars	Historical	Heimdall AAR values for a line over a time range
get_circuit_ratings	Historical	Circuit ratings for a facility over a time range

Tests

Tests are split by stability:

• Integration tests — only cover historical endpoints where the response is deterministic for a fixed time range. Tests run against the real API using the Heimdall Power Line (d67d2205-…) test fixture.
• Unit tests — cover all latest and forecast endpoints using mocks, since these can return 404 when no recent data exists. Each endpoint is tested for:
  1. 200 OK → parsed response is returned
  2. 404 Not Found → HeimdallApiError(status_code=404) is raised
  3. 404 is not retried → time.sleep is never called

New test files:

File	Type	Coverage
tests/integration/test_when_fetching_capacity_monitoring.py	Integration	get_heimdall_dlrs, get_heimdall_aars, get_circuit_ratings + invalid-ID error paths
tests/integration/test_when_fetching_grid_insights.py	Integration	get_currents, get_conductor_temperatures, get_icing, get_sag_and_clearance, get_apparent_power + invalid-ID error path
tests/unit/test_latest_endpoints.py	Unit	All 12 latest/forecast methods on HeimdallApiClient — 200 OK, 404 raised, 404 not retried
tests/unit/test_auth_service.py	Unit	Token caching, expiry, region extraction

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.NET SDK

Test fakes for WhenHandlingTransientErrors

Added fake infrastructure under WhenHandlingTransientErrors/Fakes/ to support unit testing of the HeimdallApiHttpClient when transient errors occur:

File	Description
FakeHttpMessageHandler.cs	Returns a fixed response or a sequence of responses, mirroring the one in WhenHandlingErrorResponses
HeimdallApiHttpClientFactory.cs	Factory that wires up HeimdallApiHttpClient with the fake handler and a no-op delay for instant tests
StubAccessTokenProvider.cs	Stub IAccessTokenProvider returning a static token

These follow the same pattern as the existing WhenHandlingErrorResponses/Fakes/ test infrastructure.