[FEAT]: Add resources for Enterprise Budgets

### Describe the need

[API management of Budgets](https://docs.github.com/en/enterprise-cloud@latest/rest/billing/budgets?apiVersion=2026-03-10) in now GA, and corresponding terraform resources should be created to help with large-scale management of budgets.

Proposed schema:

```hcl
resource "github_enterprise_budget" "enterprise_ai_credits" {
enterprise = "my-enterprise"
budget_amount = "300"
prevent_further_usage = true
budget_alerting = {
will_alert = true
alert_recipients = ["admin1@my-enterprise.com", "finance@my-enterprise.com"]
}
budget_scope = "multi_user_customer"
budget_entity_name = ""
budget_type = "BundlePricing"
budget_product_sku = "ai_credits"
}

data "github_enterprise_budgets" "enterprise_multi_user_customer" {
enterprise = "my-enterprise"
scope = "multi_user_customer"
}

data "github_enterprise_budgets_user_states" "enterprise_ai_50pct" {
enterprise = "my-enterprise"
budget_id = github_enterprise_budget.enterprise_ai_credits.id
# user = "joe"
threshold_lower_bound = 50
# threshold_upper_bound = 50
}
```

### GitHub Installation Type

- [ ] GitHub.com (Free, Pro, or Team)
- [ ] GitHub Enterprise Server (on-premises)
- [x] GitHub Enterprise Cloud with Personal Accounts (github.com)
- [x] GitHub Enterprise Cloud with Managed Users/EMU (github.com)
- [ ] GitHub Enterprise Cloud with Data Residency (*.ghe.com)
- [ ] I don't know

### Relevant log output

```shell

```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start with the linked GitHub Budgets REST API documentation and compare its endpoints and fields with existing Terraform provider resource and data-source patterns. Implement the proposed github_enterprise_budget, github_enterprise_budgets, and github_enterprise_budgets_user_states interfaces, with schemas matching the documented API and working Terraform plans and reads.

Written by the indexing model from the issue text.