refactor: move validation and error handling out of transport layer

[nchamo]

Why we are doing this

rpc_translator.ts and oracle.ts were mixing serialization/deserialization with validation, error handling, and orchestration logic, making it hard to reason about and difficult to auto-generate in the future.

Our fix

Validation, null checks, bounds enforcement, forbidden-operation throws, and the version compatibility check have all moved into the handler layer:

• Null-check-and-throw for six oracle methods (getNoteHashMembershipWitness, getNullifierMembershipWitness, getLowNullifierMembershipWitness,getPublicDataWitness, getBlockHeader, getAuthWitness) — handlers now return non-optional values and throw when the lookup fails.
• Forbidden operations are enforced in handler subclasses (TXEPrivateExecutionOracle, TXEOraclePublicContext) by overriding methods to throw; the transport calls the handler without knowing the difference.
• Bounds validation (MAX_PRIVATE_LOGS_PER_TX, offchain-effects length) moved into handlers.
• Decryption failure handling moved to handler (returns Buffer | undefined; transport no longer catches).
• Version compatibility check moved to TXESession.setTxeOracleVersion().
• MAX_OFFCHAIN_EFFECTS_PER_TXE_QUERY and MAX_OFFCHAIN_EFFECT_LEN moved from txe_oracle_version.ts to txe_session.ts, co-located with the validation that uses them.

The transport is now pure serialize → delegate → deserialize. No behavior change.