v2: express fixes + unit tests

[phdargen]

Description

• Implements express streaming and async route handler fixes following v1: https://github.com/coinbase/x402/pull/659, https://github.com/coinbase/x402/pull/700
• Refactors express/hono adapters following next structure: https://github.com/coinbase/x402/pull/707
• Adds unit tests to express/hono and axios/fetch

Checklist

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

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