Skills extension should prefer the canonical Foundry project endpoint

[hund030]

Problem

azure.ai.skills checks the active azd environment for AZURE_AI_PROJECT_ENDPOINT, while current Foundry project provisioning and agent initialization persist the canonical FOUNDRY_PROJECT_ENDPOINT value.

As a result, skill commands and azure.ai.skill service deployment can fail with:

no Foundry project endpoint resolved

even though other Foundry extensions resolve the current project successfully.

Expected Behavior

Skill operations should resolve the project endpoint in this order:

1. Explicit --project-endpoint
2. Active azd FOUNDRY_PROJECT_ENDPOINT
3. Active azd AZURE_AI_PROJECT_ENDPOINT compatibility fallback
4. Persisted project context
5. Process FOUNDRY_PROJECT_ENDPOINT
6. Process AZURE_AI_PROJECT_ENDPOINT compatibility fallback

Acceptance Criteria

• Skill commands work when only active azd FOUNDRY_PROJECT_ENDPOINT is configured.
• FOUNDRY_PROJECT_ENDPOINT wins when both active environment keys are set.
• Existing projects that contain only AZURE_AI_PROJECT_ENDPOINT continue to work.
• The same canonical-first behavior applies to process environment variables.
• Resolver documentation and tests reflect the supported precedence.