Flasher bundle SSRF - unvalidated client-supplied registry URL

[raballew]

Description

Client-supplied registry URL is passed to oras_client.pull() at python/packages/jumpstarter-driver-flashers/jumpstarter_driver_flashers/driver.py:72-82 with zero validation.

A lease holder can supply a malicious registry URL pointing to internal services, cloud metadata endpoints, or attacker-controlled servers.

Suggested Fix

• Add registry URL allowlist configuration parameter
• Validate scheme (HTTPS only)
• Block RFC 1918 and link-local addresses
• Pin bundles by digest
• Add @validate_call to flasher @export methods