fix: ci pnpm version mismatch and undefined headers in getBody

[karol-broda]

hey! noticed ci was failing because pnpm/action-setup was set to version 9 but package.json has packageManager set to pnpm 10.15.1.

removed the hardcoded version from the action so it reads from package.json directly.

also fixed a couple of bugs in utils.ts:

• headers variable was undefined in getBody
• getHeaders was using Object.entries on headers which breaks array-style headers like [["X-header", "1"]]

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