refactor(kms): close the low-severity follow-ups from the #5668 adversarial re-review (#5817)

* refactor(kms): share the DEK spec mapping and stop re-parsing opened envelopes

- generate_key_material is now the single spec->length mapping for every
  backend that mints DEKs itself; the inline copies in the Static and Local
  backends are gone, and ChaCha20 (32 bytes, same as AES_256) is accepted
  uniformly instead of only by Static.
- The pub(crate) client decrypt of the Local, Vault KV2 and Vault Transit
  backends returns (plaintext, master_key_id), so KmsBackend::decrypt no
  longer re-parses the envelope it just opened (one JSON parse per SSE GET
  instead of two, and unknown-field observability is no longer double-counted).
- Malformed-envelope parse failures now report CryptographicError("parse")
  on all backends; Local was the last one mapping them to SerializationError.
- The four KmsBackend::generate_data_key adapters take fields out of
  DataKeyInfo instead of cloning, dropping a redundant un-zeroized plaintext
  DEK copy and a full ciphertext clone per call; a missing plaintext now
  fails closed everywhere instead of returning an empty key on three of four
  backends.

* test(kms): pin legacy header fallback, stored-AAD, and decrypt key-id contracts

- a_legacy_aws_kms_object_without_the_cipher_header_still_opens rebuilds the
  true pre-internal-header shape (aws:kms mode + S3 key-id header, no
  x-rustfs-* headers) and asserts the fallback normalizes the cipher and
  re-projects it.
- a_rewritten_sse_c_context_header_fails_authentication is the SSE-C flank of
  the stored-AAD tamper check; metadata_without_stored_context_bytes_still_opens
  covers the derived-AAD path for both flavours and pins the seal side to the
  canonical bytes (mutation-verified).
- data_key_spec_controls_the_length_of_the_generated_key requires every
  backend in the matrix to honour all three specs, asserts the envelope
  records the requested spec, and round-trips each blob.
- corrupt_ciphertext_fails_cleanly pins unparseable ciphertext to
  CryptographicError instead of merely not-InternalError.
- Deleted the never-called assert_validation_error / assert_cryptographic_error
  helpers.

crates/kms/src/encryption/dek.rs

@@ -389,16 +389,18 @@ impl Default for AesDekCrypto {
     }
 }
 
-/// Generate random key material for the given algorithm
+/// Generate random key material for the given algorithm.
+///
+/// The lengths must track [`crate::types::KeySpec::key_size`].
 ///
 /// # Arguments
-/// * `algorithm` - The key algorithm (e.g., "AES_256", "AES_128")
+/// * `algorithm` - The key algorithm (e.g., "AES_256", "AES_128", "ChaCha20")
 ///
 /// # Returns
 /// A vector containing the generated key material
 pub fn generate_key_material(algorithm: &str) -> Result<Vec<u8>> {
     let key_size = match algorithm {
-        "AES_256" => 32,
+        "AES_256" | "ChaCha20" => 32,
         "AES_128" => 16,
         _ => return Err(KmsError::unsupported_algorithm(algorithm)),
     };