Add gmatpy smoke check via stock GMAT sample (#23)

- New src/smoke.ts exporting smoke(gmatRoot, pythonVersion?). Resolves python via @actions/io's which('python', true) — a missing binary throws a typed error pointing at actions/setup-python@v5.
- Runs python -c "..." that bootstraps gmatpy from ${gmatRoot}/bin, calls gmat.Setup against api_startup_file.txt, loads samples/Ex_HighFidelitySRP.script, and asserts gmat.RunScript returns 1 (GMAT's success code).
- @actions/exec.exec streams stdout/stderr live to the workflow log and throws on non-zero exit. Failures are wrapped with the sample path and GMAT_ROOT for the action's failure summary.
- Sample choice: Ex_HighFidelitySRP.script — smallest stock sample with Propagate, no Subscribers, header reports Released: R2014a so it ships in every supported version.
- Not yet wired into install.ts.

Closes #8

Author: djankov

src/smoke.ts

@@ -0,0 +1,58 @@
+import * as path from 'node:path';
+import * as core from '@actions/core';
+import * as exec from '@actions/exec';
+import * as io from '@actions/io';
+
+const SMOKE_SAMPLE = path.posix.join('samples', 'Ex_HighFidelitySRP.script');
+
+const SMOKE_PYTHON_SRC = `
+import os, sys
+gmat_root = os.environ['GMAT_ROOT']
+sys.path.insert(0, os.path.join(gmat_root, 'bin'))
+import gmatpy as gmat
+gmat.Setup(os.path.join(gmat_root, 'bin', 'api_startup_file.txt'))
+script = os.path.join(gmat_root, ${JSON.stringify(SMOKE_SAMPLE)})
+if not gmat.LoadScript(script):
+    sys.exit(f'gmat.LoadScript failed for {script}')
+rc = gmat.RunScript()
+if rc != 1:
+    sys.exit(f'gmat.RunScript returned {rc} for {script} (expected 1)')
+print('setup-gmat smoke ok')
+`;
+
+export async function smoke(gmatRoot: string, pythonVersion?: string): Promise<void> {
+  const pythonPath = await resolvePython();
+  const sample = path.join(gmatRoot, SMOKE_SAMPLE);
+  if (pythonVersion !== undefined) {
+    core.info(
+      `Smoke-testing GMAT install at ${gmatRoot} with ${pythonPath} (requested python-version=${pythonVersion})`,
+    );
+  } else {
+    core.info(`Smoke-testing GMAT install at ${gmatRoot} with ${pythonPath}`);
+  }
+  try {
+    await exec.exec(pythonPath, ['-c', SMOKE_PYTHON_SRC], {
+      cwd: gmatRoot,
+      env: { ...process.env, GMAT_ROOT: gmatRoot } as { [key: string]: string },
+    });
+  } catch (err) {
+    const reason = err instanceof Error ? err.message : String(err);
+    throw new Error(
+      `Smoke check failed: ${reason}. ` +
+        `Sample: ${sample}. GMAT_ROOT: ${gmatRoot}. ` +
+        `See the workflow log above for gmatpy's stderr.`,
+    );
+  }
+}
+
+async function resolvePython(): Promise<string> {
+  try {
+    return await io.which('python', true);
+  } catch (err) {
+    const reason = err instanceof Error ? err.message : String(err);
+    throw new Error(
+      `python is not on PATH (${reason}). ` +
+        `Add a 'uses: actions/setup-python@v5' step before setup-gmat.`,
+    );
+  }
+}