fix: handle MCP payment result metadata

[parvahuja]

Summary

• Parse MCP payment-required metadata from JSON-RPC tool results in the payment-aware fetch.
• Return MCP SDK payment challenges as tool results with payment metadata so SDK clients can retry through the same fetch boundary.
• Update MCP client/server tests to use returned challenge results.

Validation

• pnpm check
• VITE_TEMPO_NETWORK=none pnpm check:types
• VITE_TEMPO_NETWORK=none pnpm exec vp test --run src/client/Transport.test.ts src/mcp/server/Transport.test.ts
• pnpm build
• PATH=./node_modules/.bin:$PATH pnpm smoke

[pkg-pr-new]

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npm i https://pkg.pr.new/mppx@580

commit: e536526