Return the processed CSS from NoWorkResult#toString() - #2155

When Processor#process() gets no plugins it returns a NoWorkResult. Its
constructor runs the map generator and rewrites this.result.css: it appends a
generated sourceMappingURL annotation, or strips a stale one left in the
input. toString() skipped all of that and returned the raw input string, so
String(result) and result.css disagreed whenever the constructor had
changed anything.

lib/no-work-result.d.ts declares NoWorkResult_ implements LazyResult<Root>,
and LazyResult#toString() is documented as "Alias for the LazyResult#css
property. lazy + '' === lazy.css". LazyResult#toString() returns
this.css; NoWorkResult had css and content reading this.result.css
while toString() read the untouched this._css.

Reproduction:

const postcss = require('postcss')

// 1. a generated inline map is dropped
let a = postcss([]).process('.foo { color: red }\n', {
  from: 'foo.css',
  map: true
})
a.css      // '.foo { color: red }\n\n/*# sourceMappingURL=data:application/json;base64,... */'
String(a)  // '.foo { color: red }\n'

// 2. a stale annotation is leaked
let b = postcss([]).process(
  '.foo { color: red }\n\n/*# sourceMappingURL=old.css.map */\n',
  { from: 'foo.css', map: false }
)
b.css      // '.foo { color: red }\n'
String(b)  // the input, annotation and all

With one plugin registered the same calls take the LazyResult path and both
values agree.

The fix makes toString() return this.result.css, matching css, content
and LazyResult#toString(). This is residue from #1909, which converged the
NoWorkResult constructor with LazyResult but left toString() reading the
pre-map string.

The three new tests sit next to the no work result matches lazy result...
tests that #1909 added; those compare .css, these compare toString(). They
cover the two branches of the constructor separately: a generated inline map, a
generated external map (which also sets result.map), and a stale inline map
being stripped. The existing stringifies css test already asserted
`${result}` === result.css, but only on input the constructor never
touches, so it passed throughout.

Reverting the one-line change turns all three red; a patch that fixes only the
map-generating branch, only the annotation-stripping branch, or that trims the
result is caught too. Full pnpm test is green, 704 tests (701 before), c8
line coverage still at 100%.

Summary by CodeRabbit

• Bug Fixes

  • Stringifying a no-work result now consistently returns the processed CSS, including source map transformations, instead of the original source.

• Tests

  • Added coverage for string conversion with enabled maps, non-inlined maps, and inline source maps.