Add ResourceSnapshot customMetrics edge case tests

[sfloess]

Part of #1: Code Coverage Below Threshold

Problem

The customMetrics handling in ResourceSnapshot has important branch logic (null check in constructor, defensive copy, unmodifiable wrapper) that is untested.

Scope

Add tests covering:

• Constructor with null customMetrics -- getCustomMetrics() returns empty map
• Constructor with populated customMetrics map -- values preserved
• Defensive copy: modifying the original map after construction does not affect the snapshot
• Immutability: getCustomMetrics() returns an unmodifiable map (throws on put())

Acceptance Criteria

• All four scenarios tested
• Branch coverage for the customMetrics != null ternary in constructor
• mvn clean verify passes