fix: reject tokens missing 'aud' claim when audience is expected

[binggao1230]

Problem

jwt.decode(token, key, audience="my-service") silently accepts tokens that carry no aud claim at all. An attacker who holds a valid token from any service that omits aud can replay it against a service that checks audience, because the missing-claim path simply returns without raising.

Minimal reproduction:

from jose import jwt

token = jwt.encode({"sub": "attacker"}, "secret", algorithm="HS256")
# No 'aud' claim in the token

decoded = jwt.decode(token, "secret", algorithms=["HS256"], audience="my-service")
# Returns the claims instead of raising — cross-service replay works

PyJWT raises MissingRequiredClaimError('aud') in the same situation.

The root cause is a commented-out guard in _validate_aud (jwt.py):

if "aud" not in claims:
    # if audience:
    #     raise JWTError('Audience claim expected, but not in claims')
    return   # ← always returns, even when audience= was given

Fix

Activate the guard: raise JWTClaimsError when the caller provides a non-None audience but the token has no aud claim.

The existing options={"require_aud": True} opt-in continues to work as before for callers who need to require the claim without specifying an expected value.

Tests

• test_aud_empty_claim (which documented the wrong behavior) is replaced by two explicit cases:
  • test_aud_missing_claim_no_audience — no audience expected → still passes
  • test_aud_missing_claim_with_audience — audience given but claim missing → raises JWTError
• test_require is corrected to not pass an unrelated audience= value when testing require_<other-claim>.

All 250 tests pass (6 skipped, unchanged).

Closes #407

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This pull request was prepared with the assistance of AI, under my direction and review.