PyJWT accepts public JWK containers as HMAC secrets

### Summary PyJWT 2.13.0 contains an incomplete defense against algorithm confusion when an application mixes symmetric and asymmetric algorithms in one verification path. A public RSA, EC, or OKP JWK can be accepted as an HMAC secret when it is wrapped in a JWKS object, nested in an array, or represented in another container form that does not expose a top-level `kty` member. ### Impact An attacker who knows the public key material can forge HS256/HS384/HS512 tokens if the application simultaneously: * allows both HS* and asymmetric algorithms; * passes raw public JWK/JWKS JSON as `key=`; and * uses that same value as the HMAC secret. This can allow forged JWT claims in affected application configurations. The issue does not affect applications that keep symmetric and asymmetric verification paths separate and follow PyJWT's algorithm-selection guidance. ### Fix status The fix is on `master` in commit `801cd12` (`fix: reject public JWK container HMAC keys`). `HMACAlgorithm.prepare_key` now rejects public JWK members found in objects, arrays, nested containers, BOM/UTF variants, and recursion-limit inputs. It also recognizes escaped JSON member names without treating ordinary string values as JWKs. Ordinary JSON secrets remain accepted byte-for-byte. The change was tested with focused regression tests and the full local tox matrix. Available Python 3.9, 3.12, and 3.13 crypto/no-crypto suites, mypy, package metadata, and coverage passed; unavailable interpreters were skipped b