Consolidate overlapping srcset attribute tests in creative module

[prk-Jr]

Description

crates/common/src/creative.rs has multiple srcset tests with overlapping coverage. Consolidating them would preserve behavior while reducing maintenance overhead.

Done when

• Review the current srcset tests for overlapping cases.
• Merge redundant cases into a smaller set or parameterized structure.
• Preserve coverage for spacing, relative paths, and descriptor handling.

Affected area

• HTML processing / JS injection

Additional context

Source: test_gap.md, section Srcset Attribute Tests.
Parent issue: #396