docs: finish the remaining 5.x/4.x API reference consistency items

[bilashcse]

Closes the three remaining unchecked items in #1881.

Problem

The 4.x and 5.x API reference pages drifted apart after the redesign, which makes it hard to tell a
real API difference from a formatting difference when switching versions:

1. Result comments in code samples use double quotes in many places (// => "text/html") while the
   rest of the site, and the Prettier singleQuote setting, use single quotes.
2. The res.type() sample had a typo — res.type('png'); // => image/png: — with the value unquoted
   and a stray trailing colon (the typo 5.x docs are missing some documentation added in late 4.x versions #1881 links to).
3. req.host and req.hostname were sorted differently: 5.x lists req.host first (alphabetical,
   like every other section), 4.x lists req.hostname first.
4. The 5.x page description frontmatter said "The request object …" / "The response object …" while
   4.x and the opening sentence of both pages say "The req object …" / "The res object …".

Fix

• Normalised 33 result comments across the 4.x and 5.x application/request/response pages to single
  quotes. Double quotes are kept where the sample legitimately shows JSON (// => callback({ "user": "tobi" }))
  or a raw HTTP header (// Host: "example.com:3000").
• Fixed the res.type('png') sample to res.type('png'); // => 'image/png'.
• Moved the 4.x req.host section above req.hostname so both versions are alphabetical. I picked
  the 5.x order because every other section on the page is alphabetical — happy to flip it the other
  way if you prefer the deprecated alias to stay last.
• Aligned the 5.x request/response description frontmatter with 4.x and with the first sentence of
  each page.

No prose or behaviour was changed, only formatting, ordering and the one typo.

Tests

Added tests/unit/api-docs-style.test.mjs, which runs in the existing npm run test:unit step of
the lint job. It walks every .mdx file under src/content/api/4x and src/content/api/5x and
asserts that:

• no result comment quotes a plain string value with double quotes;
• no result comment ends with stray punctuation (this is what would have caught the image/png: typo);
• req.host is documented before req.hostname in both versions;
• the request/response description frontmatter starts with "The req object represents" /
  "The res object represents".

Each assertion fails on main before this change and passes after it.

Docs

Added a "Style Conventions" subsection to docs/content.md under API Reference describing the
three conventions and pointing at the test that enforces them, so the next person editing these pages
knows the rules without having to diff 4.x against 5.x.

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[bilashcse]

Heads up on overlap: #2487 is also open against #1881 and touches two of the same files.

Where we agree, the changes are identical (the res.type('png') typo, and the 5.x request/response description frontmatter switching to "The req object ..." / "The res object ...").

Where we differ is req.host / req.hostname. #2487 makes 5.x match 4.x: it renames the 5.x req.host section to req.hostname, drops the example.com:3000 and [::1]:3000 samples, and reports 'example.com'. That would undo a real Express 5 behaviour change, since in 5.x req.host keeps the port and req.hostname is the one that strips it. This PR instead keeps both 5.x sections as they are and moves the 4.x req.host section above req.hostname, so the ordering is alphabetical in both versions without touching what either property is documented to return.

This PR also covers the first checkbox in #1881, which #2487 does not: 33 result comments across the 4.x and 5.x application/request/response pages move from double to single quotes, plus the unit test and the docs/content.md section.

Happy to defer or rebase on whichever you would rather merge first.