Normalize file paths to absolute at execution engine entry point

[cderv]

fileExecutionEngineAndTarget in src/execute/engine.ts accepts both relative and absolute paths. This means cached ExecutionTarget.source can be either form depending on the caller, which has caused downstream bugs:

• Fix quarto preview subdir/file.qmd crashing with doubled path #14150 — projectPath doubled subdirectories when target.source was relative
• Single file rendering vs Project rendering: inconsistency in input path which is relative vs absolute  #12401 — relative vs absolute inconsistency in QUARTO_DOCUMENT_PATH and cwd computation

We could normalize file to absolute at the top of fileExecutionEngineAndTarget to establish an "always absolute" contract for target.source. This is the single convergence point for all callers (preview, render, format resolution).

quarto-cli/src/execute/engine.ts

Lines 353 to 380 in 0f97e05

	export async function fileExecutionEngineAndTarget(
	file: string,
	flags: RenderFlags | undefined,
	project: ProjectContext,
	): Promise<{ engine: ExecutionEngineInstance; target: ExecutionTarget }> {
	const cached = ensureFileInformationCache(project, file);
	if (cached && cached.engine && cached.target) {
	return { engine: cached.engine, target: cached.target };
	}
	// Get the launched engine
	const engine = await fileExecutionEngine(file, flags, project);
	if (!engine) {
	throw new Error("Can't determine execution engine for " + file);
	}
	const markdown = await project.resolveFullMarkdownForFile(engine, file);
	const target = await engine.target(file, flags?.quiet, markdown);
	if (!target) {
	throw new Error("Can't determine execution target for " + file);
	}
	// Cache the ExecutionEngineInstance
	cached.engine = engine;
	cached.target = target;
	return { engine, target };
	}

Just registering this idea to look into it when we are ready to make things more robust.