Fix MPP fetch

[Haakam21]

Summary by cubic

Fixes MPP/x402 fetch by using the client's raw fetch for the initial 402 probe, avoiding signed requests and enabling correct credential creation. Aligns with ENG-300 (SDKs x402/MPP).

• Bug Fixes
  • probe402 now accepts an optional fetchFn and uses it.
  • getPaymentCredentials passes mppx.rawFetch to probe402.
  • Updated unit tests to mock rawFetch.

^{Written for commit 3e77ef9. Summary will update on new commits.}

[cubic-dev-ai]

cubic analysis

No issues found across 4 files

Linked issue analysis

Linked issue: ENG-300: SDKs x402/MPP

Status	Acceptance criteria	Notes
✅	probe402 accepts an optional fetch function and uses it	Added fetchFn param and used fetchFn(httpUrl) in util.ts
✅	getPaymentCredentials passes mppx.rawFetch to probe402	getPaymentCredentials now calls probe402(wsUrl, mppx.rawFetch)
✅	MppxClient interface includes a rawFetch property	Raw fetch added to MppxClient interface in mppx.ts
✅	Avoid using signed requests for the initial 402 probe (use client's raw fetch)	probe402 now accepts injected fetch and getPaymentCredentials passes rawFetch
✅	Unit tests updated to mock rawFetch where applicable	Tests were modified to include rawFetch mocks in two test files
⚠️	Enable correct credential creation (response used to create credentials)	Response passed to createCredential but no test verifies credential correctness