Surface meaningful skip reasons when no Azure VM size meets test requirements

[Copilot]

When no Azure VM size satisfies a test case's requirements, LISA previously emitted a generic "no available quota found" message, making it hard to diagnose why a test was skipped.

Changes

• _get_meet_capabilities: Changed item from tuple-unpacking to list indexing so unmet check_result.reasons from the first failing candidate can be written back to item[2] after the generator is exhausted.
• _get_azure_capabilities: After determining no VM was found, collects and de-duplicates the stored unmet_reasons across all candidate items. Produces either a specific "Requirement mismatch: <reasons>" message or a structured fallback noting the cause is undiagnosed.

Before / After

# Before
no available quota found on 'eastus'.

# After — when requirements are diagnosable
Requirement mismatch: gpu_count minimum is 1, but capability is 0; nic_count minimum is 4, but capability is 2

# After — when cause is unknown
Test skipped on 'eastus' for an unknown reason. This could be due to insufficient quota,
unmet hardware requirements, or other undiagnosed causes. Manual investigation may be required.

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