fix: handle 404s in NodeResolver.php

[justlevine]

What does this implement/fix? Explain your changes.

This PR fixes a bug where NodeResolver::resolve_uri() falls back to the home page when trying to resolve a non-existent URI.

This issue is due to our refactor of resolve_uri() to use WP_Query (#2680), which didnt take into account that our existing parse_request() shim deviated from WP::parse_request().

With the PR, parse_request() now correctly sets the $query_vars['error'] value in line with WP core. Additionally, queries directly to the home page ('/'), are resolved early. This increases performance, but more importantly addresses the fact that most WP sites dont have an explicit redirect rule for the home page (which causes $query_vars['error'] to be set for those requests as well.

To Test.

1. Create a post and a page.
2. (Optional) Set your permalink structure to something custom e.g. /posts/%postname%/.
3. Run the following query (change the URIs to what you created.

fragment NodeData on Node {
  __typename
  id
  ... on UniformResourceIdentifiable {
    uri
  }
  ... on ContentType {
    name
  }
  ... on NodeWithTitle{
    title
  }
}
query {
  firstNonExistingPage: nodeByUri(uri:"/fake-page") {
    ...NodeData
  }
	existingPage: nodeByUri(uri:"/home-page") {
    ...NodeData
  }
  secondNonExistingPage: nodeByUri(uri:"/another-fake-page") {
    ...NodeData
  }
}

Does this close any currently open issues?

Fixes #2751

Any relevant logs, error output, GraphiQL screenshots, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Any other comments?

• This does not resolve a separate issue where $this->wp can become stale when there are multiple nodeByUri queries in the same request. I cannot replicate that bug consistently enough to address it, it preexists nodeByUri resolving incorrectly when permalinks are set to a custom structure #2751, and does not interfere with this fix, so I don't believe it should be considered a blocker for either this PR or v1.14.0.
• This does not include an explicit test for #nodeByUri resolving incorrectly when permalinks are set to a custom structure #2751 . @jasonbahl feel free to add one if you think its necessary.
  Where has this been tested?

---

Operating System: Ubuntu 20.04 (wsl2 + devilbox + php 8.0.26)

WordPress Version: 6.1.1

[coveralls]

Coverage: 84.873% (+0.05%) from 84.825% when pulling f7315e1 on justlevine:fix/handle-404s-in-node-resolver into ead0acb on wp-graphql:develop.

[codeclimate]

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