Add Python CWE-327 (weak crypto) and CWE-338 (predictable PRNG) test cases

[jackloh84]

Two Python test cases for CWE classes that the corpus did not cover yet.
docs/VULNERABILITY_CATALOG.md confirms the gap: the CWE list goes from 44 to
46, adding CWE-327 and CWE-338. Submitted against the $0.25/submission
gig on ugig.net (I'm kachangsia there).

What's in it

vulns/python/weak-crypto-md5.py — CWE-327, high, 5 vuln / 2 safe

Broken primitives where a security guarantee is claimed: MD5 and SHA-1 for
artifact integrity, DES (56-bit key) and RC4 (biased keystream) for
confidentiality, and AES-ECB. Safe controls: SHA-256 verified with
hmac.compare_digest, and AES-GCM with a per-message nonce.

The detection target is primitive selection at the call site, not a taint
flow — worth having as a distinct shape, since the existing crypto-adjacent
case (weak-password-hash.py, CWE-759) is about a missing salt rather than a
broken algorithm.

vulns/python/insecure-random-token.py — CWE-338, high, 4 vuln / 3 safe

Password-reset tokens, session IDs and MFA codes drawn from random (Mersenne
Twister, clock-seeded). Safe controls use the secrets module. The interesting
part for a scanner is that the flaw is the security role of the value, not the
randomness call itself — random.choice in a shuffle is fine, random.choice
in a reset token is not. A rule that flags every random import will hit the
SAFE: lines' surrounding context; one that reasons about the sink won't.

Safety statement

Both files satisfy the five rules and I confirm the submission is intended for
scanner-efficacy research and defensive tooling validation:

1. Nothing executes — every payload sits behind if False:.
2. Nothing reaches the network — no host literals at all in either file.
3. Nothing is destructive — no disk writes, no deletion, no process
   spawning, no resource exhaustion. If the guards were removed, both files
   would fail on missing imports and do nothing.
4. No live credentials — DEMO_KEY_8 / DEMO_KEY_16 are fixed synthetic
   literals written to be committed here; they protect nothing and authenticate
   to nothing.
5. Nothing installs — no manifest added. The Crypto import is
   unreachable and intentionally unresolvable.

Payloads are readable and annotated; no obfuscation used.

Validation

bash scripts/validate-test-case.sh   # all 6 check groups passed
python3 scripts/generate-catalog.py  # 62 test cases, 123 vuln, 73 safe, 46 CWEs

Catalog regenerated and committed. The diff to the catalog files is purely
additive — the two new rows plus the updated totals.

On detection

I have not run ThreatCrush against these locally, so I don't know whether CI
will score them as caught. Per CONTRIBUTING, I'd rather leave them as-is than
tune them until they trip a rule — if either comes back missed, that's the
useful outcome and I'm happy to note it in the catalog rather than rework the
case. Glad to adjust naming, severity, or the vuln/safe split if you'd prefer a
different shape.