fix(node-sdk): declare @x402/fetch as an optional peer dependency

[rohitsux]

Fixes agentmail-to/agentmail-node#24.

Problem

The node SDK's wrapper lazily imports @x402/fetch (src/wrapper/x402.ts, src/wrapper/Client.ts) for the opt-in payments feature, but the package is not declared anywhere in package.json. As the issue lays out, that phantom dependency breaks bundlers (unresolvable-import errors), file tracers such as @vercel/nft (a configured external must be resolvable on disk), and isolated installers (pnpm, bun), forcing consumers to carry externalization workarounds. The wrapper's own src/wrapper/x402-types.d.ts module shim exists precisely because TypeScript cannot resolve the undeclared package.

Why the fix is here and not in agentmail-node

package.json in agentmail-node is Fern-generated and not listed in .fernignore, so declaring the dependency there directly would be overwritten on the next regeneration. The generator config in this repo is the durable layer: the packageJson block's fields (description, keywords, license) already flow into the published package, and the TypeScript generator supports extraPeerDependencies / extraPeerDependenciesMeta for exactly this case.

Change

extraPeerDependencies:
  "@x402/fetch": "^2"
extraPeerDependenciesMeta:
  "@x402/fetch":
    optional: true

This keeps payments opt-in (consumers install @x402/fetch only if they use the feature) while making the import visible to every toolchain. All published @x402/fetch versions are 2.x, and the wrapper uses only x402HTTPClient and wrapFetchWithPayment, both stable across 2.x.

Verification

npx fern-api check: 0 errors, 9 warnings — identical to unmodified main (the warnings are pre-existing).

Follow-up (out of scope here)

Once this is released, src/wrapper/x402-types.d.ts in agentmail-node could be dropped in favor of the real types from the now-resolvable package.