fix: validate requests_per_second

[worlldz]

Summary

• reject requests_per_second values below 1 during provider configuration validation
• add a reusable minimum-value validator without introducing a new dependency
• cover null, unknown, invalid, boundary, and positive values through the provider schema

Why

The provider currently passes requests_per_second directly to rate.NewLimiter as both the rate and burst size. Values of zero or less create a limiter that cannot service a request, so an accepted Terraform configuration fails later during the first API call with a rate limiter error.

Validating the value in the provider schema gives users an immediate, attribute-specific diagnostic while preserving the existing default of 5 requests per second.

Testing

• go test ./... -run Test -skip TestAcc
• go build .
• go vet ./...
• go tool staticcheck ./...