Preflight: warn-only on quota for the Bicep variant

[achandmsft]

Fixes #23. Follow-up to #21 / #22.

After the previous fix, deploying the Bicep variant with quota exhausted produced two near-identical quota warnings — one from the preflight script, one from azd's own provisionParametersResolver. Pamela hit this on eastus2 after #22:

(!) Warning: Insufficient quota for model "claude-sonnet-4-6" (SKU: GlobalStandard) in eastus2
  Requested: 25 · Available: -20
  Suggestion: No quota is available. Change your deployment location via azd env set AZURE_LOCATION <location> ...

That second message is azd doing its own ARM preflight and prompting the customer to continue. For Bicep, it's the source of truth. The preflight's hard fail just blocks that better UX.

What changes

Both preflight scripts now detect the IaC variant from ./azure.yaml:

Variant	Quota check
provider: bicep	warn + continue (exit 0) — azd's ARM preflight surfaces it next
provider: terraform	unchanged — hard fail (exit 6) because azapi_resource swallows quota into the opaque 400 715-123420
can't determine (script run outside a variant folder)	hard fail — safe default

The marketplace-offer check (exit 4) stays a hard fail in both variants.

Verification

Same CLAUDE_SONNET_CAPACITY=999 request against eastus2 (limit 80):

• From infra-terraform/:

  ERROR: Insufficient quota for 'claude-sonnet-4-6' ... exit=6
  

• From infra-bicep/:

  Preflight WARNING: Insufficient quota for 'claude-sonnet-4-6' ...
  (Continuing — azd's Bicep ARM preflight will repeat this and prompt to continue.)
  Preflight OK.
  exit=0
  

skills/claude-on-foundry/SKILL.md and both scripts' header comments are updated to describe the new behavior.