fix: decouple shared REST utilities from Axios and verify tree-shaking

[urbankrepel-povio]

Shared REST utility declarations imported Axios types even for native consumers. Remove those imports, accept unknown errors in message matching, and support native Headers as well as Axios-style header records.

Preserve every existing root export, including RestClient, RestInterceptor, and their types. Consumers can continue using those imports; no migration is required. Native consumers can tree-shake unused Axios exports in a bundle or use the dedicated subpaths for direct runtime imports.

Add an isolated consumer check to CI covering:

• Direct non-Axios subpath imports and strict declaration compilation with Axios absent.
• A bundled native/shared-helper import from the root with Axios absent: inspect the output for Axios imports and execute the resulting bundle.
• Legacy root exports and types with Axios installed, plus the dedicated Axios entry point.

Boundary: unbundled root imports still load Axios, and root declarations retain the Axios-specific types for compatibility. Rolldown warns about the absent peer before treating it as external and eliminating the unused import; this check does not establish missing-peer resolution behavior for every bundler.

Local validation: build, typecheck, lint, optional-peer consumer checks, tree-shaking checks, and both full test modes. CI results are separate from local checks.