Refactor UTF-8 boundary tests in streaming_replacer

[prk-Jr]

Description

crates/common/src/streaming_replacer.rs contains several UTF-8 boundary tests with overlapping intent and setup, which makes the suite harder to maintain.

Done when

• Replace overlapping UTF-8 boundary tests with a table-driven or otherwise consolidated structure.
• Preserve coverage for multibyte and emoji boundary handling.
• Keep the refactor limited to test organization, not runtime behavior changes.

Affected area

• Core (synthetic IDs, cookies, GDPR)

Additional context

Source: test_gap.md, section UTF-8 Boundary Tests.
Parent issue: #396