feat: add TFE support for get_policy_details tool

[fphilippon]

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

Context

This PR introduces full support for retrieving individual Sentinel policy details from Terraform Enterprise (TFE) and Terraform Cloud (TFC), while maintaining compatibility with existing Terraform Registry functionality.

Before this change, it was not possible to fetch individual policy details from TFE/TFC.
The existing get_policy_details tool only supported: Public Terraform Registry (policies/...)

This created a gap for enterprise users who need to:

• Inspect individual policies
• Understand policy logic and enforcement
• Build tooling on top of policy introspection (e.g., AI assistants)

What has been implemented:

• Extended get_policy_details to support Terraform Enterprise / Terraform Cloud (TFE/TFC) in addition to the public Terraform Registry

• Implemented auto-detection logic based on ID prefix:
  • pol-... → fetch via TFE/TFC API
  • policies/... → fetch via Terraform Registry

• Added getTFEPolicyDetails() to retrieve individual Sentinel policy details from TFE/TFC

• Introduced new tool get_policy_set_details to fetch policy set metadata (polset-...) including policies, workspaces, and enforcement levels

• Registered new tool and added dynamic availability checks based on TFE authentication

• Ensured backward compatibility with existing registry functionality

• Added test coverage for new TFE-related features

[hashicorp-cla-app]

All committers have signed the CLA.