Add wsh debugterm (#2947)

`wsh debugterm` currently decodes terminal bytes sourced from backend
block files. This extends it with a stdin-driven path so FE-emitted
payloads like `["...", "..."]` can be decoded directly without requiring
block lookup/RPC in that mode.

- **CLI surface**
  - Added new mode: `--mode stdin`
  - Updated mode help text to: `hex`, `decode`, `stdin`
  - Centralized mode validation via `getDebugTermMode()`

- **Execution path split**
  - Added mode-aware pre-run (`debugTermPreRun`):
    - `stdin` mode: skips RPC setup
    - `hex`/`decode`: keeps existing RPC setup behavior
  - `stdin` mode now:
    - reads all stdin
    - parses JSON as `[]string`
    - concatenates entries and runs existing decode formatter

- **Parsing support**
  - Added `parseDebugTermStdinData([]byte) ([]byte, error)`
  - Error messaging explicitly requires a JSON array of strings

- **Tests**
  - Added focused coverage for:
    - valid stdin JSON array parsing + decoded output
    - invalid stdin JSON input

```go
stdinData, _ := io.ReadAll(WrappedStdin)
termData, err := parseDebugTermStdinData(stdinData) // expects []string JSON
if err != nil { return err }
WriteStdout("%s", formatDebugTermDecode(termData))
```

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: sawka <2722291+sawka@users.noreply.github.com>
Co-authored-by: sawka <mike@commandline.dev>

Author: Copilot