Preserve headers and request body in fetch client

[phdargen]

Description

A collection of fixes for the v2 fetch client:

• allow optional fetch request init config as in fix(x402-fetch): allow optional fetch request init config #660 for v1 by @0xRAG
• preserve headers object during 402 retry, v1 PR fix: preserve Headers object during 402 retry in x402-fetch #314 by @dankelleher
• fixes "request body is already used" error reported in Bugs in x402-fetch #752, similar to Fix x402-fetch #753 by @juchiast that targeted v1

Tests

📊 Test Summary
==============
✅ Passed: 2
❌ Failed: 0
📈 Total: 2

📋 Detailed Test Results
========================

✅ PASSED TESTS:

  # 1: fetch → express → /protected
      Facilitator: typescript | Network: eip155:84532 | Tx: 0x5060992a...
  # 2: fetch → express → /protected-svm
      Facilitator: typescript | Network: solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1 | Tx: LKzj1HZaH3...

📊 Breakdown by Facilitator:
   typescript      ✅ 2 / ❌ 0 (100%)

📊 Breakdown by Server:
   express              ✅ 2 / ❌ 0 (100%)

📊 Breakdown by Client:
   fetch                ✅ 2 / ❌ 0 (100%)

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

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