fix: align memory embedding provider catalog - #7472

Summary

• align the Memory embedding-provider catalog with runtime auto-detection support
• keep provider, model, and API-key environment selections compatible when the provider changes
• keep custom model entry stable while the user types
• persist Auto-detect selections as explicit null clears and remove the corresponding TOML overrides server-side
• return canonical cleared/default values from the PATCH response and cover the behavior with route and UI regressions

Verification

• corepack pnpm exec vitest run src/pages/Memory/constants.test.ts src/pages/Memory/index.test.tsx (2 files, 15 tests passed)
• corepack pnpm typecheck
• corepack pnpm lint
• CARGO_TARGET_DIR=/private/tmp/librefang-target-pr7472 cargo test -p librefang-api --lib routes::memory::tests::optional_string_patch_removes_explicit_nulls -- --exact
• CARGO_TARGET_DIR=/private/tmp/librefang-target-pr7472 cargo test -p librefang-api --test memory_routes_integration patch_memory_config_null_clears_embedding_overrides -- --exact
• CARGO_TARGET_DIR=/private/tmp/librefang-target-pr7472 cargo clippy -p librefang-api --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

Out of scope

• adding new runtime embedding providers or changing backend provider defaults
• existing Polish and Ukrainian plural-key parity drift