fix: Apply extension-contributed project metadata in quarto preview

[mcanouil]

Fixes #14783.

Project metadata contributed by an extension through contributes.metadata.project, such as brand, was applied on the render path only.
The preview command builds the project context before the render services exist, so the contribution never reached the project configuration.

The merge now runs for every project context, with a fresh extension context when the render options are absent.
f6a0f89 made that same move for engine extensions in #14208, and left the metadata merge behind the gate with the note that it needs the render options.
The merge in fact uses services.extension only, so this change completes that work instead of taking a different route to it.
Invalid contributed metadata stays fatal for a render, and becomes a warning that names the extension elsewhere, so a faulty extension cannot stop quarto preview, quarto inspect, quarto publish or quarto serve.
Every command that builds a project context applies the metadata now, so quarto inspect reports it as well.

The new tests build the context the way preview does, without render options, and cover the brand for a single file and for a project, the render path, a contributed output-dir, and a faulty extension.
Run locally with the brand, extension, project and inspect tests, 55 tests, all correct.

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