Route fill through native credential broker

[jmcte]

Summary

• routes Swift broker fill requests through the same injected credential broker path as login
• preserves the request intent in successful broker responses for both login and fill
• expands Swift coverage for fill failure mapping, non-demo credentials.json isolation, and AuthenticationServices broker routing
• collapses SDK-specific ASAuthorizationError cases into the stable unknown broker code and documents the wire-contract behavior

Related issue: #13.

This does not complete the notarized hardware validation tracked by #43.

Verification

• bash scripts/ci/run-fast-checks.sh - passed
• swift test --package-path native-app - passed, 17 tests
• bash scripts/build-native-app.sh - passed
• cargo test --manifest-path rust/Cargo.toml --test native_app_e2e -- --test-threads=1 - passed, 12 tests
• git diff --check - passed

Notes

• The sandboxed native_app_e2e run could not bind the fake broker UNIX socket (PermissionError: Operation not permitted). The same suite passed outside the sandbox.