Wire TME test subscription through azd config to preflight pipeline

[jongio]

Problem

Functional/playback tests need Azure subscription, tenant, and location. Currently developers must manually set env vars (AZD_TEST_AZURE_SUBSCRIPTION_ID, etc.) each session. The non-CI fallback only covers subscription/location via defaults.*, not tenant, and conflates personal defaults with test config.

Proposal

Add defaults.test.* keys to azd config:

azd config set defaults.test.subscription 4d042dc6-fe17-4698-a23f-ec6a8d1e98f4
azd config set defaults.test.tenant 70a036f6-8e4d-4615-bad6-149c02e7720d
azd config set defaults.test.location eastus2

Changes needed

1. cli/azd/test/functional/cli_test.go - Update cliConfig.init() resolution order: env vars -> defaults.test.* -> defaults.*
2. cli/azd/magefile.go - Read defaults.test.* from azd config and pass as env vars to test subprocesses (unit + playback)
3. azd-preflight skill - Document that azd config set defaults.test.* is the recommended one-time setup for local preflight with live test resources

Developer experience

One-time setup:

azd config set defaults.test.subscription 4d042dc6-...
azd config set defaults.test.tenant 70a036f6-...
azd config set defaults.test.location eastus2

Then mage preflight and the azd-preflight skill automatically pick up the TME subscription - no env vars, no .env files.

[jongio]

Related to epic #7660 — improves test execution reliability for the preflight pipeline.