#123 [High] Add null-handling and ingestion tests for `Token.priceUsd` across the tokens module

[james2177]

Description: prisma/schema.prisma's Token model defines priceUsd Float? as explicitly nullable ("Latest known USD price (nullable – updated by a price-feed worker)"), and issue #55 backs the Token model with a real repository/service, but neither src/tokens/tokens.service.ts nor src/tokens/tokens.controller.ts currently has verified, tested behavior for the null case — before a price-feed worker (issue #8's pluggable price-feed provider) has ever populated a token's price.

Problem Statement & Context: Between issue #55 (real Token repository) and issue #8 (pluggable price-feed provider) landing, there's a real window — and likely a permanent steady-state for newly-listed tokens — where priceUsd is null. Any consumer (quote calculation, RoutingService, API responses) that assumes a numeric price without checking will produce NaN/crash rather than a clear "price unavailable" signal.

Scope & Acceptance Criteria:

• Audit every read site of Token.priceUsd (via src/tokens/tokens.service.ts and any downstream consumer such as quote-calculation logic) and ensure each handles null explicitly (typed as number | null throughout, never silently coerced).
• Add a documented, explicit behavior for API consumers when priceUsd is null (e.g. the field is omitted vs. returned as null in the response — pick one and make it consistent, then document it in Swagger per the pattern in issue Add an audit trail for cancelled and expired intents #62).
• Out of scope: implementing the price-feed worker itself (issue feat: port /api/v1/intents routes to Nest #8) — this issue only hardens the null-handling path that exists regardless of when/whether a price feed is populating data.
• Out of scope: TokensService.getByChain()'s separately-tracked return-shape bug (issue feat: port /api/v1/stats endpoint to Nest #9).

Implementation Guidelines:

1. Key files: prisma/schema.prisma (Token.priceUsd), src/tokens/tokens.service.ts, src/tokens/tokens.controller.ts, src/tokens/tokens.data.ts (current static data source, useful for comparing pre/post issue Replace SolversService's in-memory Map with persistent storage #55 shapes).
2. Trace any arithmetic performed on priceUsd (e.g. USD-value calculations feeding stats/quote logic) and guard each with an explicit null check rather than relying on TypeScript's optional-chaining alone to prevent a runtime NaN.
3. Coordinate the null-vs-omitted API contract decision with whatever Swagger documentation issue Add an audit trail for cancelled and expired intents #62 adds for TokensController, so the two stay consistent.
4. Testing: minimum 90% branch coverage on the null-handling paths specifically; add test cases for a token with priceUsd: null at every read site identified in the audit, plus a regression test confirming no NaN/unhandled-exception path exists.

Definition of Done:

• Null-handling audited and hardened across all identified read sites, tested, documented.
• Acceptance criteria met.
• PR passes CI with test output.
• Reviewed and approved.

Resources: prisma/schema.prisma, src/tokens/tokens.service.ts, src/tokens/tokens.controller.ts, src/tokens/tokens.data.ts

Complexity: High (200 points)