Preserve OHTTP/HPKE error granularity in FFI bindings

[chavic]

OHTTP/HPKE errors are flattened into generic errors. Clients can’t distinguish “invalid key length” from network/encapsulation failures.

Todo:

• Export granular error enums for OHTTP/HPKE into FFI.
• Preserve parse/decode vs transport vs crypto errors.
• Add negative tests in each binding to assert variant specificity.
• Keep backwards compatibility by preserving generic error as a top‑level wrapper.

[spacebear21]

Closing as wontfix per my comment here #1431 (review)