Close host-testable test-coverage gaps (tracked by #285) #298

Closes the host-testable test-coverage gaps identified in the assessment spec docs/superpowers/specs/2026-06-16-test-coverage-gap.md.

Adds and hardens unit tests only — no source-logic changes — across the following areas:

• edgezero-core/src/handler.rs — DynHandler::call + IntoHandler bridge (body flow-through + error propagation)
• edgezero-cli/src/bin/check_no_nested_app_config.rs — nested-AppConfig CI checker: all type_contains recursion arms (Rc/Arc/tuple/array/slice/ref/paren + combos), struct_derives cases, and an end-to-end NestedAppConfigVisitor suite (15 tests)
• edgezero-adapter/src/registry.rs — AdapterPushContext builder + trait no-op defaults + error-returning push_config_entries[_local] defaults
• edgezero-adapter/src/cli_support.rs — run_native_cli not-found / non-zero-exit paths
• edgezero-core/src/compression.rs — gzip/brotli decode error paths
• edgezero-macros/src/app.rs — build_route_tokens error + happy paths

All five project CI gates pass: cargo fmt, cargo clippy -D warnings, cargo test --workspace, feature check, and the wasm32 Spin check.

Addressed by #285.