Fix timeout telemetry truth

[mylukin]

Summary

• abort provider/pipeline work with the gateway request timeout signal across API surfaces
• persist client-visible gateway timeouts as final error/timeout instead of late provider ok
• bump root package version to 0.25.15 for release publishing

Validation

• pnpm exec vitest run apps/gateway/src/middleware/limits.test.ts apps/gateway/src/routes/chat.route.test.ts apps/gateway/src/routes/messages.test.ts apps/gateway/src/routes/responses.test.ts apps/gateway/src/routes/gemini.test.ts apps/gateway/src/routes/images.test.ts apps/gateway/src/routes/interactions.test.ts apps/gateway/src/routes/payload-capture.test.ts apps/gateway/src/app.test.ts --config vitest.config.ts
• pnpm --filter @helm/gateway typecheck
• pnpm lint
• pnpm --filter @helm/gateway build
• pnpm build