Manage projectcontext objects correctly on preview

[cscheid]

This should close a few of our quarto preview intermittent issues whose symptom is a missing quarto_ipynb file.

See, eg, #12992.

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[cscheid]

This last failing test is interesting. If I'm reading the messages correctly, we previously had:

• a test that used to pass but which requested invalid options
• a code path that verified the validity of options that wasn't getting called because of incorrect plumbing of the ProjectContext object

Now that things are properly connected, the test fails. But it seems that this test shouldn't really even be there?

[cderv]

The test failure in render-output-dir.test.ts (and the other playwright docs/playwright/embed-resources/issue-11860/main.qmd) is caused by a logic ordering issue at:

quarto-cli/src/command/render/render-shared.ts

Lines 51 to 63 in e48007d

	let context = pContext || (await projectContext(path, nbContext, options)) ||
	(await singleFileProjectContext(path, nbContext, options));
	// Create a synthetic project when --output-dir is used without a project file
	// This creates a temporary .quarto directory to manage the render, which must
	// be fully cleaned up afterward to avoid leaving debris (see #9745)
	if (!context && options.flags?.outputDir) {
	context = await projectContextForDirectory(path, nbContext, options);
	// forceClean signals this is a synthetic project that needs full cleanup
	// including removing the .quarto scratch directory after rendering (#13625)
	options.forceClean = options.flags.clean !== false;
	}

The change on lines 51-52 now always creates a singleFileProjectContext as fallback:

let context = pContext || (await projectContext(path, nbContext, options)) ||
(await singleFileProjectContext(path, nbContext, options));

This means context is never null when the --output-dir check runs on line 57:

if (!context && options.flags?.outputDir) {

The synthetic project creation for --output-dir is now unreachable. The fix could be to check for --output-dir before falling back to singleFileProjectContext, if we really need this fallback. I am unsure about that and its relation to preview fix which is the topic of this PR.

For example

diff --git a/src/command/render/render-shared.ts b/src/command/render/render-shared.ts
--- a/src/command/render/render-shared.ts
+++ b/src/command/render/render-shared.ts
@@ -48,9 +48,7 @@ export async function render(
   const nbContext = pContext?.notebookContext || notebookContext();

   // determine target context/files
-  // let context = await projectContext(path, nbContext, options);
-  let context = pContext || (await projectContext(path, nbContext, options)) ||
-    (await singleFileProjectContext(path, nbContext, options));
+  let context = pContext || (await projectContext(path, nbContext, options));

   // Create a synthetic project when --output-dir is used without a project file
   // This creates a temporary .quarto directory to manage the render, which must
@@ -62,6 +60,11 @@ export async function render(
     options.forceClean = options.flags.clean !== false;
   }

+  // Only fall back to singleFileProjectContext if we don't have a context yet
+  if (!context) {
+    context = await singleFileProjectContext(path, nbContext, options);
+  }
+
   // set env var if requested
   if (context && options.setProjectDir) {
     // FIXME we can't set environment variables like this with asyncs flying around