outboundVnetRouting.managedIdentityTraffic undocumented behavior causes Managed Identity token acquisition failure when allTraffic=true

### Description

When using Azure Function App with VNET integration and forced tunneling, Managed Identity token acquisition fails unless the undocumented property
`outboundVnetRouting.managedIdentityTraffic` is explicitly set to false.

This property is not consistently documented and its default behavior is unclear, leading to broken Managed Identity authentication in secure network designs.

### Scenario

- Azure Function App
- System-assigned Managed Identity enabled
- Regional VNET Integration
- outboundVnetRouting.allTraffic = true
- Subnet has UDR (e.g. 0.0.0.0/0 → NVA / Firewall)

### Expected behavior

Managed Identity token issuance should work as long as IMDS is reachable (as per existing documentation).

### Actual behavior

Managed Identity token issuance fails unless:

`outboundVnetRouting.managedIdentityTraffic = false`

is explicitly set.

When set to `false`, token acquisition works immediately without any other changes.

### Observed errors

- ManagedIdentityCredential authentication unavailable
- Failed to acquire MSI token
- Timeouts when calling IMDS / IDENTITY_ENDPOINT
- SQL / Key Vault authentication failures that appear unrelated to networking