rpc: fix SendRawTransaction_Result.Reason json tag, add tag-hygiene tests

[DHEBP]

SendRawTransaction_Result.Reason is tagged json:"string" instead of
json:"reason", so every DERO.SendRawTransaction response carries an
undocumented "string" key and a client reading "reason" finds nothing.

Impact today is nil rather than data loss. Reason is never assigned anywhere in
the tree, so the field is always empty. The two soft-failure paths in
rpc_dero_sendrawtransaction.go report via result.Status, and real rejections
return a Go error that becomes a JSON-RPC error object, so the failure text a
client actually sees is unaffected either way.

I corrected the tag rather than removing the field. Whether to populate it or
delete it is an API decision for maintainers, and populating it would mean
changing the rejection branch to return a result instead of an error -- that
removes the JSON-RPC error object which is currently the only rejection signal
derohe's own wallet reads, so it is a breaking change and should not ride along
with a tag fix.

The second half is a guard so this class does not recur. rpc/jsontag_test.go
parses the package source (rather than using reflection) and asserts:

• no json tag named after a Go builtin type -- this is what the defect above
  looks like mechanically: the field's TYPE written where its NAME belongs
• no duplicate json tag within a struct, where one field silently never
  reaches the wire and which one wins depends on field order
• json tags are lowercase, matching the rest of the wire format

It runs under go test ./rpc/. No new dependency and no CI job required.

Verification: the first check fails on the unfixed tree, naming the exact
file:line and suggesting the correct tag, and passes after the fix. The other
two pass in both states, so the suite is not merely failing everything. I
re-confirmed this after rebasing by reintroducing the bad tag and watching the
check fire. go build ./... is clean and go test ./rpc/ passes.

Every type in this package is part of derod's or the wallet's public wire
format, and a wrong tag there is invisible at compile time and invisible to
tests that only exercise Go structs -- it only shows up as a field a client can
never read.