Gate Azure integration tests on explicit deploy context

[Copilot]

azure_infrastructure_test.py crashed with subscription_id must not be None on any machine without Azure context, making the validation suite non-reproducible and unusable in CI without a live subscription.

Changes

• conftest.py (new): Registers the azure_integration pytest marker and hooks pytest_collection_modifyitems to skip all tagged tests when AZURE_SUBSCRIPTION_ID is absent — with a message that names the missing vars and points to docs.

• azure_infrastructure_test.py:

  • Adds pytestmark = pytest.mark.azure_integration to opt all 30 tests into the gate.
  • Replaces module-level credential = DefaultAzureCredential() with a lazy _get_credential() so import no longer probes Azure credentials.
  • Adds a __main__ preflight that exits 2 with actionable steps when AZURE_SUBSCRIPTION_ID is unset.

• docs/SETUP.md: New Running Integration Tests section covering required env vars, pytest and standalone command paths, and an expected-outcomes table for each environment state.

Behavior

# Clean machine — no Azure context
$ pytest azure_infrastructure_test.py -rs
SKIPPED [30] — missing env vars: AZURE_SUBSCRIPTION_ID. Set them and run `az login`...

$ python azure_infrastructure_test.py
ERROR: Azure deploy context is not configured.
Missing required environment variables: AZURE_SUBSCRIPTION_ID
...
Exit code: 2

# With context — tests run normally
$ export AZURE_SUBSCRIPTION_ID=$(az account show --query id -o tsv)
$ pytest azure_infrastructure_test.py -v