fix(providers): model list empty in settings panel (#581)

[kagura-agent]

Problem

Settings panel model dropdown shows empty list even though providers are properly configured in openclaw.json and the gateway works fine for chat. Closes #581.

Root Cause

Two issues in ProviderService.listAccounts():

1. Destructive cleanup on empty config: When getActiveOpenClawProviders() returned an empty Set (transient condition during gateway restart, config file lock, first launch before config sync), the code deleted all provider accounts from the electron-store, including builtin ones. Next time the settings panel loaded — empty list.

2. No seeding from OpenClaw config: Users who configured providers via CLI or openclaw.json directly (not through ClawX UI) had no corresponding ProviderAccount entries in the electron-store. The settings panel only reads from the store, so it showed nothing.

Fix

Defensive cleanup (commits 1-2)

• When activeProviders.size === 0, skip cleanup entirely and return existing accounts as-is (with logger.warn() for diagnostics)
• Move builtin-provider skip (if (isBuiltin) continue) earlier in the loop — builtin accounts should never be candidates for deletion

Config seeding (commit 3)

• When the electron-store is empty but openclaw.json has configured providers, seed ProviderAccount entries from the config
• New getOpenClawProvidersConfig() reads the raw provider config
• New seedAccountsFromOpenClawConfig() creates account entries with correct vendor IDs, base URLs, and inferred model names
• One-time operation — subsequent calls find existing accounts and skip seeding

Tests

5 unit tests covering:

1. Preserves all accounts when activeProviders is empty (defensive path)
2. Removes stale non-builtin accounts when config has active providers (normal cleanup)
3. Never removes builtin providers even if not in activeProviders
4. Returns empty and calls getActiveOpenClawProviders when account list is empty (seed check)
5. Matches accounts by vendorId, id, or openClawKey

All 254 existing tests pass.