serviceability: validate device mgmt_vrf field

[ben-dz]

Summary

• Validate the device mgmt_vrf field in Device::validate: restrict it to the account-code charset ([A-Za-z0-9:_-]) and cap it at 32 bytes, matching how the device code is already bounded. Empty is still accepted (the default VRF).
• The controller skips rendering device config when a string field destined for the device config contains characters that wouldn't survive as a single config token (control characters or whitespace), failing closed instead of emitting a malformed config.
• Move setup_device_prerequisites from device_onchain_allocation_test.rs into test_helpers.rs so the new test reuses it.

Testing Verification

• Audited mgmt_vrf across all existing devices on mainnet-beta, testnet, and devnet — every value ("", default, MGMT, mgmt, management) passes the new rule, so the Device::validate check applies cleanly to deployed accounts.
• New onchain integration test: CreateDevice/UpdateDevice reject out-of-charset (Custom(19)) and over-length (Custom(34)) values, a failed update leaves the stored value unchanged, and empty/valid values round-trip.
• New controller render test covers all 13 rendered string fields and asserts the error names the offending field.