sidecar: harden /tx?hash= discriminator and lock ctx-plumbing audit

[bdchatham]

Closes #172 items 1 and 2. Item 3 (fee_payer/granter integration assertion) ships with the gov-vote handler PR — it makes more sense alongside a handler that actually signs a tx end-to-end.

Summary

• Replace strings.Contains(strings.ToLower(err.Error()), \"not found\") with a typed *rpctypes.RPCError discriminator (isTxNotFound). Transport errors are wrapped Go errors, not *RPCError, so errors.As fences them out — DNS "host not found" can no longer be confused with a missing tx.
• Lock the ctx-plumbing audit (item 2) in a code comment. The vendored sei-tendermint jsonrpc client uses http.NewRequestWithContext, so the 30s tmRPCTimeout from feat(sidecar): sign-tx foundation (PR-A of #163, foundation-only) #170 is a redundant upper bound, not the only stop.

Test plan

• Unit test TestIsTxNotFound covers: server-side tx-not-found, other server internal error, wrapped RPCError, transport error containing "not found" as decoy substring, nil
• `go build ./...` clean
• `go test ./...` clean
• `go test -race ./sidecar/tasks/...` clean
• `golangci-lint run --new-from-rev=origin/main ./...` reports 0 issues
• Coral trio cross-review (platform / kubernetes / security) — all clean, k8s independently verified the ctx-plumbing claim

🤖 Generated with Claude Code

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Note

Low Risk
Low risk: small, well-tested change to error classification in QueryTx plus a clarifying comment about context cancellation; main impact is how missing-tx vs real transport errors are handled.

Overview
Tightens QueryTx error handling by replacing the substring-based "not found" check with a typed *rpctypes.RPCError discriminator (isTxNotFound), preventing transport/DNS errors from being misclassified as missing transactions.

Adds a unit test suite for isTxNotFound (including wrapped RPC errors and decoy "not found" transport messages) and documents that ctx cancellation propagates into the underlying Tendermint JSON-RPC HTTP request (making the existing 30s timeout a secondary bound).

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