--account N was silently ignored when UNSANDBOX_PUBLIC_KEY/SECRET_KEY
env vars were set. The credential resolution checked env vars at tier 2
before ever reaching the CSV lookup, so the explicit flag had no effect.

Correct priority order (all 8 SDKs):
  1. CLI -p/-k flags (explicit key args)
  2. --account N → direct CSV row lookup (bypasses env vars)
  3. UNSANDBOX_PUBLIC_KEY / UNSANDBOX_SECRET_KEY env vars
  4. ~/.unsandbox/accounts.csv default (row 0 or UNSANDBOX_ACCOUNT)

SDKs updated: C, Python, Go, JavaScript, Ruby, PHP, Java, Rust

Also adds:
- --account N flag to CLI parsers in all 8 SDKs (Go, PHP, Java, Rust
  previously had no flag at all; Python/JS/Ruby had the parameter but
  never wired it to the CLI)
- test_account_flag.sh integration test for each SDK verifying the
  priority behavior with real and garbage credentials
- test-integration Makefile target for the C SDK