ci: trigger CI after openapi spec updates + fix generator nullable type

[natalie-o-perret]

Two changes:

1. CI: .github/workflows/main.yml triggers CI on completion of Update bundled openapi definition (the hourly job that pushes spec updates to master), plus an explicit master entry in push.branches. A bad spec update now fails CI instead of going silent.
2. Generator: exoscale/api/generator.py adds _resolve_type() to handle OpenAPI 3.1 nullable schemas like "type": ["integer", "null"]. The old code assumed schema["type"] was a string and died at import with unhashable type: 'list', bricking the whole SDK. This is the breakage the CI hook above is meant to catch next time.

Verified on Python 3.10 to 3.14: 16 passed on every version. Output:

tests/test_client.py::test_client_creation PASSED                        [  6%]
tests/test_client.py::test_client_error_handling PASSED                  [ 12%]
tests/test_client.py::test_wait_interval PASSED                          [ 18%]
tests/test_client.py::test_operation_poll_failure PASSED                 [ 25%]
tests/test_client.py::test_operation_invalid_state PASSED                [ 31%]
tests/test_client.py::test_wait_time_success PASSED                      [ 37%]
tests/test_client.py::test_wait_time_poll_errors PASSED                  [ 43%]
tests/test_client.py::test_wait_time_failure PASSED                      [ 50%]
tests/test_partner_client.py::test_client_creation PASSED                [ 56%]
tests/test_partner_client.py::test_client_with_custom_url PASSED         [ 62%]
tests/test_partner_client.py::test_client_error_handling PASSED          [ 68%]
tests/test_partner_client.py::test_authentication PASSED                 [ 75%]
tests/test_partner_client.py::test_create_organization_with_structured_address PASSED [ 81%]
tests/test_partner_client.py::test_create_organization_with_optional_fields PASSED [ 87%]
tests/test_partner_client.py::test_create_organization_minimal_billing_address PASSED [ 93%]
tests/test_partner_client.py::test_list_organizations PASSED             [100%]

============================== 16 passed in 0.06s ==============================

Note

AI-assisted for patch draft and this very PR description.

[natalie-o-perret]

[SC-179745]