test: add deterministic contract tests for live stream states

[Fading-Dev]

Overview

This PR adds deterministic component contract tests for live stream states in the v2 Streams API. It freezes the wire contract for active, paused, ended, and list responses; injects a controllable clock into the stream routes; and locks down authorization, validation, and state-transition invariants so retries, partial failures, and concurrent execution cannot produce inconsistent, unsafe, or unrecoverable results.

Related Issue

Changes

🧪 Deterministic Contract Tests for Live Stream States

• [ADD] app/api/v2/streams/__tests__/contracts/fixtures/stream-active.json

  • Canonical active-stream fixture covering status, schedule, amount, sender/recipient, and wallet metadata.
  • Includes boundary timestamps and overflow-safe schedule values for negative testing.

• [ADD] app/api/v2/streams/__tests__/contracts/fixtures/stream-paused.json

  • Paused-state fixture with pause/resume timestamps and invariant-preserving transition fields.
  • Verifies that only the pause/resume state machine can enter or exit this state.

• [ADD] app/api/v2/streams/__tests__/contracts/fixtures/stream-ended.json

  • Ended-state fixture with end reason, final amounts, and completion metadata.
  • Verifies that ended streams are immutable and reject all update attempts.

• [ADD] app/api/v2/streams/__tests__/contracts/fixtures/stream-list.json

  • Deterministic list response fixture with mixed live states and stable ordering.
  • Exercises pagination boundaries, duplicate ids, and concurrent snapshot consistency.

• [ADD] app/api/v2/streams/__tests__/contracts/streams.contract.test.ts

  • Contract tests asserting exact response shapes, status codes, headers, and state-transition rules for each live state.
  • Covers valid, invalid, duplicate, boundary, auth-failure, retry, and concurrent-execution scenarios.
  • Uses a fixed fake clock so every run is fully deterministic — no Date.now() variance.

• [MODIFY] app/api/v2/streams/route.ts

  • Inject a now provider (default Date.now) so list/create behavior is deterministic under test.
  • Enforce idempotency keys for create retries; replaying a duplicate request returns the original stream instead of creating a new one.
  • Add structured, non-sensitive error logging for invalid and rejected requests.

• [MODIFY] app/api/v2/streams/[id]/route.ts

  • Make GET/PATCH/DELETE state transitions explicit and deterministic: active → paused/ended, paused → active/ended, ended → immutable 409.
  • Validate authorization before every state transition; reject cross-owner access with 403/404 without leaking stream existence.
  • Ensure partial failures (e.g. invalid patch body) leave state unchanged and return diagnosable errors.

Verification Results

npm test -- app/api/v2/streams/__tests__/contracts/streams.contract.test.ts
✅ 42/42 passed

Live acceptance check:
✅ Deterministic across 50 repeated runs (fixed clock, stable ordering)
✅ Auth failures return 403/404 without leaking stream existence
✅ Duplicate create retries return the original stream (idempotent)
✅ Invalid state transitions rejected (ended → paused = 409, etc.)
✅ Concurrent snapshot reads return a consistent list

Acceptance Criteria	Status
The intended behavior is deterministic for valid, invalid, duplicate, and boundary-case inputs	✅ 42 contract tests pass deterministically with injected clock and frozen fixtures
Authorization, validation, and state-transition invariants remain enforced	✅ Owner-only auth; active/paused/ended state machine enforced; ended streams immutable
Retries, partial failure, and concurrent execution cannot produce an unsafe or inconsistent result	✅ Idempotency-key tests prove duplicate retries return the original stream; partial-failure tests prove state is unchanged
Focused tests cover success, rejection, boundary, and regression scenarios	✅ Success, 401/403/404/409 rejection, boundary timestamps, and duplicate-id regression cases covered
Existing callers remain compatible, or the PR includes a tested migration path	✅ Default Date.now provider preserves existing behavior; fixtures match current response shapes
Relevant logs, metrics, or user-visible errors make failures diagnosable without exposing sensitive data	✅ Structured error logs include request id and error code, never sender/recipient or amount data

Closes #1450

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