fix(cli): warn when --data fields cannot be sent

[ysyneu]

Problem

--data bodies are bound into the SDK request struct with json.Unmarshal, which silently discards fields the struct does not declare. When the server accepts a field that the bundled SDK does not know yet (or the caller mistypes a key), the field disappears before the request is sent and the command still exits 0.

Change

• genBindBody takes an io.Writer and, after binding, walks the body against the request type (unboundFields / wireField, following encoding/json name matching: exact then case-insensitive, untagged embedded structs, url-tagged GET structs, stopping at types with their own UnmarshalJSON).
• Every dropped path is named on stderr, e.g.
  warning: --data fields not sent, unknown to this CLI version: investigation_targets[].bogus
• The request is unchanged, so read-modify-write flows that pass extra read-only fields keep working.
• cligen template and hand-written callers pass ctx.Cmd.ErrOrStderr(); the insight export's second bind of the same body (incident total) passes io.Discard.
• Generated files regenerated with go run ./internal/cmd/cligen (only the call line changes).

Verification

• go test ./... passes; make check-cards OK; new unit test covers nested objects, arrays, maps, embedded structs, url tags, case folding and json.RawMessage.
• Built binary against a local HTTP stub with monit rule-v2-create --data carrying investigation_targets[].purpose plus an unknown key: purpose is in the outgoing body; the unknown key is reported on stderr.

Also supersedes #223: main already carries go-flashduty v0.15.15 (which includes the purpose field) via #224.