Tests: add Azure fixture strategy/scaffolding for future remote-store support

[charles-turner-bot]

Problem

Azure support is also incomplete/unsupported in parts of the builder/catalog path, and there is currently no concrete fixture strategy for testing it when support improves.

Scope

Add Azure-oriented test fixture scaffolding so future Azure support can be implemented against something real instead of staying entirely abstract.

This may include:

• deciding whether to use Azurite, disposable test containers, or another documented integration path
• fixture helpers for account/container setup and teardown
• placeholder tests covering current unsupported/error behaviour
• clear notes on which config is safe to serialise vs must remain runtime-only

Acceptance criteria

• There is a documented Azure fixture/test strategy.
• Current unsupported behaviour is captured in tests where practical.
• Future Azure implementation work has a clear place to add real round-trip coverage.

Related

• Supports Enhancement: generalise builder/catalog remote-store handling beyond local filesystem #33 and Enhancement: define the supported builder source-format matrix (including opendap status) #35.