feat: implement pre-release guard functionality with tests

Author: CasperKristiansson

docs/tasks.md

@@ -139,7 +139,7 @@ Use Context7 MCP for up to date documentation.
         If already at latest, set `skipped_reason=already_latest`.
         Verify: Second run produces skip.
 
-17. [ ] **Pre-release guard**
+17. [x] **Pre-release guard**
         Default off. Enabled by `include_prerelease=true`.
         Verify: Tests confirm behavior.
 

src/versioning/index.ts

@@ -6,3 +6,6 @@ export type { PythonOrgFallbackOptions, PythonOrgVersion } from './python-org-fa
 
 export { fetchRunnerAvailability } from './runner-availability';
 export type { RunnerAvailability, RunnerAvailabilityOptions } from './runner-availability';
+
+export { enforcePreReleaseGuard } from './pre-release-guard';
+export type { PreReleaseGuardResult } from './pre-release-guard';

src/versioning/pre-release-guard.ts

@@ -0,0 +1,27 @@
+import semver from 'semver';
+
+export interface PreReleaseGuardResult {
+  allowed: boolean;
+  reason?: 'pre_release_guarded';
+}
+
+export function enforcePreReleaseGuard(
+  includePrerelease: boolean,
+  version: string | null,
+): PreReleaseGuardResult {
+  if (version === null) {
+    return { allowed: true };
+  }
+
+  const parsed = semver.parse(version, { includePrerelease: true, loose: true });
+  if (!parsed) {
+    return { allowed: true };
+  }
+
+  const isPreRelease = parsed.prerelease.length > 0;
+  if (isPreRelease && !includePrerelease) {
+    return { allowed: false, reason: 'pre_release_guarded' };
+  }
+
+  return { allowed: true };
+}

tests/pre-release-guard.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from 'vitest';
+
+import { enforcePreReleaseGuard } from '../src/versioning';
+
+describe('enforcePreReleaseGuard', () => {
+  it('blocks pre-release versions when flag is false', () => {
+    const result = enforcePreReleaseGuard(false, '3.13.0-rc.1');
+
+    expect(result.allowed).toBe(false);
+    expect(result.reason).toBe('pre_release_guarded');
+  });
+
+  it('allows pre-release versions when flag is true', () => {
+    const result = enforcePreReleaseGuard(true, '3.13.0-rc.1');
+
+    expect(result.allowed).toBe(true);
+    expect(result.reason).toBeUndefined();
+  });
+
+  it('allows stable versions regardless of flag', () => {
+    const result = enforcePreReleaseGuard(false, '3.13.2');
+
+    expect(result.allowed).toBe(true);
+  });
+
+  it('allows when version is null', () => {
+    const result = enforcePreReleaseGuard(false, null);
+
+    expect(result.allowed).toBe(true);
+  });
+});