Preserve backend failures in transparent and transaction RPCs

[skazistp-cpu]

Fixes #606. Preserve backend failures instead of reporting successful zero balances/empty UTXOs or a missing transaction. Unclassified transparent RPC errors return Unknown; GetTransaction preserves cancellation/deadline status and reserves NotFound for the structured missing-transaction error, including darksidewalletd.

Regression coverage uses real in-memory gRPC, the HTTP JSON-RPC adapter, and the darkside backend. Jarvis used Codex for implementation and tests; Lagertha contributed the transaction-error lead. Exact-commit independent AI review and validation are recorded in the follow-up comment; no separate human review is implied.

[skazistp-cpu]

Validation and AI review for 65a405f67ecacc22768b9357bcf6dc1688573132:

• Full go test -count=1 ./..., full go test -race -count=1 ./..., and go build ./... pass on macOS arm64 with Go1.26.8. The independent runs each report137 passing test/subtest events across seven tested packages, with no failures or skips.
• The original transparent-RPC suite covers all four unary/streaming variants: eight cases fail on unchanged upstream and16 controls pass; all24 pass with this fix. The original production patch was also checked with Remove zcashd support #580's helper rename.
• The transaction suite exercises the real HTTP JSON-RPC adapter through in-memory gRPC: internal backend errors, warm-up, HTTP failures, wrapped cancellation/deadline and connection failure no longer become NotFound. Seven new cases fail before the correction; four controls preserve missing/valid/malformed transaction and invalid-input behavior.
• A real DarksideInit/DarksideReset regression preserves missing-transaction NotFound. An independent AI reviewer found this compatibility gap in the first candidate; its probe was incorporated into the committed regression and the revised exact commit was reviewed again.

Independent AI helper /root/lightwalletd_commit_review returned Tested ACK for the exact commit above after fresh full standard/race tests and snapshot verification, with no remaining actionable findings. This is AI review, not a separate human reviewer or approval by our shared GitHub account.

Lagertha supplied the GetTransaction lead; Jarvis used Codex for the correction and tests. This extends the existing backend-error correction. Backend inputs are local test servers/stubs; no live node or wallet was used. Docker image testing was not performed because Docker is unavailable locally.