Add comprehensive tests for remote manifest support

Adds cmd/common_test.go with 51 test cases covering:
- isRemoteFile() URL detection (16 test cases)
  - HTTP/HTTPS URLs with ports, query params, fragments
  - File paths (absolute, relative, parent directories)
  - Invalid URLs and edge cases

- isPrivateIP() SSRF protection (21 test cases)
  - IPv4/IPv6 loopback and link-local addresses
  - Private IP ranges (10.x, 172.16-31.x, 192.168.x, fc00::, fd00::)
  - Public IPs and edge cases

- downloadRemoteFile() with mock HTTP servers (14 test cases)
  - Success/error scenarios (404, 500)
  - Size limit enforcement (Content-Length and actual size)
  - YAML validation and document limits
  - Redirect handling with SSRF protection
  - Timeout handling (30s)
  - Various Content-Type headers

Co-Authored-By: Claude <[email protected]>
Signed-off-by: Pradipta Banerjee <[email protected]>

Author: bpradipt