Refactor overlapping test coverage for streaming and creative rewrites

[ChristianPavilonis]

Summary

• Refactor overlapping UTF-8 boundary tests in streaming_replacer into shared helpers and consolidated coverage-focused cases.
• Consolidate repetitive srcset and imagesrcset creative rewrite tests into shared helpers plus table-driven case matrices.
• Keep runtime behavior unchanged while reducing duplicated test setup and preserving multibyte, emoji, spacing, relative path, and descriptor coverage.

Changes

File	Change
crates/trusted-server-core/src/streaming_replacer.rs	Added reusable chunk-processing test helpers and replaced overlapping UTF-8 boundary tests with consolidated case-based coverage, including explicit multibyte and emoji boundary splits.
crates/trusted-server-core/src/creative.rs	Added shared srcset/imagesrcset test helpers and merged overlapping attribute rewrite tests into compact table-driven cases while keeping integration-specific tests separate.

Closes

Closes #457
Closes #458

Test plan

• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• JS tests: cd crates/js/lib && npx vitest run
• JS format: cd crates/js/lib && npm run format
• Docs format: cd docs && npm run format
• WASM build: cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve
• Other: Focused Rust test runs for the new consolidated test cases

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses tracing macros (not println!)
• New code has tests
• No secrets or credentials committed