test: cover holdings without price snapshot

[bamiebot-maker]

Summary

• return total_value: "0" when a holding's creator has no price snapshot
• preserve the holding and its key_count quantity
• add service-level missing-snapshot coverage and endpoint/controller integration coverage
• verify the endpoint responds successfully without forwarding an error

Verification

• pnpm test -- --runInBand src/modules/wallets/wallet-holdings-price-snapshot.integration.test.ts src/modules/wallets/__tests__/wallet-holdings.integration.test.ts ? 13 passed
• pnpm lint ? passed
• pnpm build ? passed
• pnpm exec prettier --check for all changed files ? passed after formatting

The repository-wide Jest invocation exceeded 10 minutes locally; the focused suites completed cleanly.

Closes #634

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