Fix crash on invalid agent cards without supported_interfaces

[a6b8]

Problem

When an A2A agent card is fetched successfully (HTTP 200, valid JSON) but is missing supported_interfaces, SnapshotBuilder.build() crashes on .map() of undefined. This causes the entire validation promise to reject instead of returning structured error messages.

Root Cause

• CapabilityClassifier.classify() calls supportedInterfaces.map() without null-check
• SnapshotBuilder.build() calls supportedInterfaces.map() and skills.map() without null-checks
• A2aAgentValidator.start() always calls classify/build even when CardStructureValidator reports structure errors

Fix

1. Early-return in start() when structure validation fails — use buildEmpty() instead of build()
2. Defensive Array.isArray() checks in SnapshotBuilder.build() for supported_interfaces and skills
3. Defensive Array.isArray() check in CapabilityClassifier.classify() for supported_interfaces