fix: replicate the original action's test-project auto-detection

Real gap caught by live CI (Builds - MacOS's 6000.0.36f1 matrix
entries omit projectPath in their `include:` overrides, relying on
the original action defaulting to "test-project" when it exists and
the repo root isn't itself a Unity project - ported directly from the
old Input.projectPath getter). Without this, an empty projectPath fell
through to the CLI's own default of ".", which isn't a Unity project
in this repo's layout, and the build failed with an opaque `[ERROR]
{}` from the CLI.

resolve-project-path.ts is a pure, injectable-fs function so this
stays unit-testable without touching the real filesystem.

src/resolve-project-path.test.ts

@@ -0,0 +1,53 @@
+import { describe, it, expect } from 'vitest';
+import { resolveProjectPath } from './resolve-project-path';
+
+function fsOf(existingPaths: string[]) {
+  return {
+    joinPath: (...segments: string[]) => segments.join('/'),
+    existsSync: (candidate: string) => existingPaths.includes(candidate),
+  };
+}
+
+describe('resolveProjectPath', () => {
+  it('returns the explicit input unchanged when given', () => {
+    const result = resolveProjectPath({
+      input: 'my-project',
+      ...fsOf(['test-project/ProjectSettings/ProjectVersion.txt']),
+    });
+
+    expect(result).toBe('my-project');
+  });
+
+  it('strips a trailing slash from an explicit input', () => {
+    const result = resolveProjectPath({ input: 'my-project/', ...fsOf([]) });
+
+    expect(result).toBe('my-project');
+  });
+
+  it('defaults to "test-project" when it looks like a Unity project and the repo root does not', () => {
+    const result = resolveProjectPath({
+      input: '',
+      ...fsOf(['test-project/ProjectSettings/ProjectVersion.txt']),
+    });
+
+    expect(result).toBe('test-project');
+  });
+
+  it('does not default to "test-project" when the repo root is itself a Unity project', () => {
+    const result = resolveProjectPath({
+      input: '',
+      ...fsOf([
+        'test-project/ProjectSettings/ProjectVersion.txt',
+        'ProjectSettings/ProjectVersion.txt',
+      ]),
+    });
+
+    expect(result).toBe('');
+  });
+
+  it('returns empty (letting the cli default to ".") when neither location looks like a Unity project', () => {
+    const result = resolveProjectPath({ input: '', ...fsOf([]) });
+
+    expect(result).toBe('');
+  });
+});