btcjson: prevent infinite recursion in StringOrArray.MarshalJSON and …

[cvieira-tng]

Fix #2369

Change Description

Fix an infinite-recursion bug in btcjson.StringOrArray.MarshalJSON that
caused a stack-overflow panic at runtime.

• Switch receiver from value to pointer so we no longer call the same method
  on a copy of the slice (which re-enters indefinitely).
• Add unit tests (TestGetNetworkInfoWarnings) to cover single-string and
  slice cases as well as malformed input.
• No external API changes.

Closes #.

---

Steps to Test

1. Build & test

   go test ./...          # all packages pass  

2. **Reproduce the old panic (optional)**

   ```bash
   git checkout HEAD^            # commit before this PR  
   go test -run TestGetNetworkInfoWarnings ./btcjson  # panic  
   ```

   Returning to this PR’s head makes the test suite pass again.

---

## Pull Request Checklist

### Testing

* [x] Your PR passes all CI checks.
* [x] Tests covering the positive and negative (error paths) are included.
* [x] Bug fixes contain tests triggering the bug to prevent regressions.

### Code Style and Documentation

* [x] The change is not
  [[insubstantial](https://github.com/btcsuite/btcd/blob/master/docs/code_contribution_guidelines.md#substantial-contributions-only)](https://github.com/btcsuite/btcd/blob/master/docs/code_contribution_guidelines.md#substantial-contributions-only).
* [x] The change obeys the
  [[Code Documentation and Commenting](https://github.com/btcsuite/btcd/blob/master/docs/code_contribution_guidelines.md#code-documentation-and-commenting)](https://github.com/btcsuite/btcd/blob/master/docs/code_contribution_guidelines.md#code-documentation-and-commenting)
  guidelines, and lines wrap at 80.
* [x] Commits follow the
  [[Ideal Git Commit Structure](https://github.com/btcsuite/btcd/blob/master/docs/code_contribution_guidelines.md#model-git-commit-messages)](https://github.com/btcsuite/btcd/blob/master/docs/code_contribution_guidelines.md#model-git-commit-messages).
* [x] Any new logging statements use an appropriate subsystem and logging
  level.