Haoxuanxu/add retrieve approval policy route

[haoxuanxu-stripe]

Summary

Add Approval Policy retrieval support to link-cli and @stripe/link-sdk, aligned with the server contract from [mint#2599662](https://git.corp.stripe.com/stripe-internal/mint/
pull/2599662).

• Add link-cli approval-policy retrieve
• Add Link.approvalPolicy.retrieve() to the SDK
• Call GET /approval-policy
• Model spend_request_create rules with required per-purchase limits and optional allowed payment methods
• Surface errors when no policy is configured
• Update documentation and package changesets
• Remove the obsolete, unreleased Spending Policy implementation

Response examples

Configured policy:

{
  "rules": [
    {
      "action": "spend_request_create",
      "limits": {
        "per_purchase": {
          "amount": 5000,
          "currency": "usd"
        }
      },
      "allowed_payment_methods": ["balance"]
    }
  ]
}

An explicitly configured empty policy:

{
  "rules": []
}

No configured policy returns HTTP 404:

{
  "error": {
    "message": "No approval policy has been configured",
    "code": "approval_policy_not_found"
  }
}

Motivation

The original implementation targeted the Spending Policy API from mint#2586017. That API was subsequently replaced by Approval Policy, and /spending-policy now intentionally returns
404.

Test plan

• SDK: 177 tests passing across 16 test files
• CLI: 424 tests passing across 36 test files
• SDK and CLI type checks passing
• SDK and CLI package builds passing
• Verified root help advertises approval-policy
• Verified success, unauthenticated, invalid-response, feature-unavailable, and policy-not-configured cases

Rollout/revert plan

The server endpoint is already deployed behind enable_approval_policy_api. This is an unreleased client feature, so no compatibility alias or migration is required. Safe to revert.