CI runtime: disable selected workflows from draft PRs

[padelsbach]

Prevent the following workflows from running on draft PRs.

**Skipped on draft PRs** (runs when marked ready):

| Workflow                                    | Job(s)                    |
|---------------------------------------------|---------------------------|
| Build and Test                              | all                       |
| Build and Test Refactor                     | build (macos-latest) only |
| Build and Run Examples                      | all                       |
| Build and Test Client-Only                  | all                       |
| Multi-threaded Stress Test                  | all                       |
| whnvmtool build and test                    | all                       |
| Build with installed wolfSSL (WOLFSSL_LIB)  | all                       |
| Benchmark                                   | all                       |
| Code Coverage                               | all                       |
| Code Coverage Refactor                      | all                       |

For drafts, the workflows that actually run are:

• Build and Test Refactor (ubuntu-latest): ~18 min
• Clang Format Check
• Static Analysis
  ~24 runner-min total.

This is another good-citizen change. Runner minutes per push drops from approx 140 to 25. Today draft PRs take up approx 18-25% of runner minutes.

Remaining tests are kicked off as soon as the PR is marked ready.