scaladoc: support expectations in snippets

[bracevac]

Fixes #25647

This adds compiler-suite-style diagnostic expectations for checked scaladoc snippets.

Supported:

• // error
• // warn
• repeated markers on one line, e.g. // error // error

How to enable it:

• enable compile+test for the markdown file in scaladoc's snippetCompiler / -snippet-compiler configuration
• example: docs/_docs/reference/experimental/capture-checking/basics.md=compile+test

How to use it:

• annotate expected diagnostics directly in Scala snippets

In compile+test mode:

• clean snippets must compile without diagnostics
• failing snippets must mark expected errors
• warning snippets must mark expected warnings
• unannotated errors or warnings fail

Not ported:

• // anypos-error
  Useful in the compiler test suite for awkward parser/scanner cases, but not a good fit for scaladoc.
• // nopos-error / // nopos-warn
  Position-less diagnostics don't arise in self-contained documentation snippets (they occur in multi-file macro scenarios), so these annotations have no practical use here.
• full compiler-output checking with sc-check:<id>
  A poor fit for docs, since it checks wrapped compiler output rather than the focused excerpts we want to show in documentation.

How much have you relied on LLM-based tools in this contribution?

A lot.

How was the solution tested?

Automated tests, plus manual testing on the CC language reference.