feat: add configurable MCP transport security settings via gns3_server.conf

Add MCP transport security configuration to gns3_server.conf with permissive
defaults that align with GNS3's design philosophy and VM distribution requirements.

## Changes

### 1. Configuration Schema (gns3server/schemas/config.py)
- Added MCP transport security fields to ServerSettings class:
  - mcp_enable_dns_rebinding_protection (bool, default: True)
  - mcp_allowed_hosts (list[str], default: ["*"])
  - mcp_allowed_origins (list[str], default: ["*"])
- Added field validators to handle comma-separated string input

### 2. MCP Server Initialization (gns3server/api/routes/mcp/__init__.py)
- Import TransportSecuritySettings from mcp.server.transport_security
- Added _create_mcp_server() function to read configuration
- Updated FastMCP instantiation to use configured security settings

### 3. Configuration Sample (gns3server/config_samples/gns3_server.conf)
- Added MCP transport security settings section
- Documented default behavior and security options
- Provided examples for different use cases

## Design Philosophy

**Default: Allow All Hosts** (matches GNS3's 0.0.0.0 binding):
- VM distribution works out-of-the-box
- Users can access from any network location
- Security-conscious users can restrict when needed

**Security: Optional Restriction**:
Users can configure specific hosts for enhanced security:
``ini
mcp_allowed_hosts = 127.0.0.1:*,localhost:*,192.168.1.3:*
mcp_allowed_origins = http://127.0.0.1:*,http://localhost:*,http://192.168.1.3:*
```

## Benefits

- Flexible: Users can configure based on security requirements
- User-friendly: Default matches GNS3's 0.0.0.0 binding philosophy
- Maintainable: No code changes needed for different deployment scenarios
- Secure: DNS rebinding protection remains enabled with configurable hosts

## Related

- Issue #2771
- FastMCP DNS rebinding protection design
- Existing skills configuration in ServerSettings

gns3server/api/routes/mcp/__init__.py

@@ -39,6 +39,7 @@ from fastapi.responses import Response
 from pydantic import Field
 
 from mcp.server.fastmcp import FastMCP
+from mcp.server.transport_security import TransportSecuritySettings
 
 from gns3server.config import Config
 from gns3server.services import auth_service
@@ -101,7 +102,22 @@ def _server_url() -> str:
 
 # ── FastMCP Server ────────────────────────────────────────────────────
 
-mcp = FastMCP("GNS3 MCP Server")
+def _create_mcp_server() -> FastMCP:
+    """Create MCP server with security settings from configuration."""
+    cfg = Config.instance().settings.Server
+
+    mcp = FastMCP(
+        "GNS3 MCP Server",
+        transport_security=TransportSecuritySettings(
+            enable_dns_rebinding_protection=cfg.mcp_enable_dns_rebinding_protection,
+            allowed_hosts=cfg.mcp_allowed_hosts,
+            allowed_origins=cfg.mcp_allowed_origins,
+        ),
+    )
+    return mcp
+
+
+mcp = _create_mcp_server()
 
 
 # ── Tool handlers ─────────────────────────────────────────────────────