Fix single-file preview handler using project handler with wrong default

The handler selection at line 251 had the same project-truthiness bug
as the URL path computation: single-file projects used
projectHtmlFileRequestHandler (defaultFile="index.html") instead of
htmlFileRequestHandler (defaultFile=basename of output). This caused
GET / to return 404 for single-file previews since v1.9.

Add isSingleFile guard to handler selection. Also fix test cleanup
by letting createMockProjectContext own temp dir creation.

Author: cderv

src/command/preview/preview.ts

@@ -248,7 +248,7 @@ export async function preview(
       changeHandler.render,
       project,
     )
-    : project
+    : project && !project.isSingleFile
     ? projectHtmlFileRequestHandler(
       project,
       normalizePath(file),

tests/unit/preview-initial-path.test.ts

@@ -10,15 +10,13 @@
 import { unitTest } from "../test.ts";
 import { assertEquals } from "testing/asserts";
 import { join } from "../../src/deno_ral/path.ts";
-import { normalizePath } from "../../src/core/path.ts";
 import { previewInitialPath } from "../../src/command/preview/preview.ts";
 import { createMockProjectContext } from "./project/utils.ts";
 
 // deno-lint-ignore require-await
 unitTest("previewInitialPath - single file returns empty path (#14298)", async () => {
-  const dir = normalizePath(Deno.makeTempDirSync({ prefix: "quarto-test" }));
-  const outputFile = join(dir, "hello.html");
-  const project = createMockProjectContext({ dir, isSingleFile: true });
+  const project = createMockProjectContext({ isSingleFile: true });
+  const outputFile = join(project.dir, "hello.html");
 
   const result = previewInitialPath(outputFile, project);
   assertEquals(result, "", "Single-file preview should use root path, not filename");
@@ -28,9 +26,8 @@ unitTest("previewInitialPath - single file returns empty path (#14298)", async (
 
 // deno-lint-ignore require-await
 unitTest("previewInitialPath - project file returns relative path", async () => {
-  const dir = normalizePath(Deno.makeTempDirSync({ prefix: "quarto-test" }));
-  const outputFile = join(dir, "chapter.html");
-  const project = createMockProjectContext({ dir });
+  const project = createMockProjectContext();
+  const outputFile = join(project.dir, "chapter.html");
 
   const result = previewInitialPath(outputFile, project);
   assertEquals(result, "chapter.html", "Project preview should include relative path");
@@ -40,9 +37,8 @@ unitTest("previewInitialPath - project file returns relative path", async () =>
 
 // deno-lint-ignore require-await
 unitTest("previewInitialPath - project subdir returns relative path", async () => {
-  const dir = normalizePath(Deno.makeTempDirSync({ prefix: "quarto-test" }));
-  const outputFile = join(dir, "pages", "about.html");
-  const project = createMockProjectContext({ dir });
+  const project = createMockProjectContext();
+  const outputFile = join(project.dir, "pages", "about.html");
 
   const result = previewInitialPath(outputFile, project);
   assertEquals(result, "pages/about.html", "Project preview should include subdirectory path");