fix: backslash newlines when copying from a code block (#2709)

* fix: backslash newlines when copying from a code block

When copying inline content from inside a code block, the text/plain
clipboard payload had a backslash before every newline (markdown's
hard-break syntax) and the text/html had `<br>` separators inside the
code instead of literal newlines.

Two changes fix this:

- copyExtension routes selections inside a code block through the
  block-export path so the code block's own toExternalHTML produces
  the proper `<pre><code>` wrapper.
- serializeInlineContentExternalHTML now plumbs blockType through to
  inlineContentToNodes (mirroring the internal HTML serializer) so
  `\n` in code-block content stays as literal text instead of being
  split into hardBreak nodes that render as `<br>`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix: drop `as const` from PRETTIFY_OPTIONS to satisfy htmlfy types

`as const` typed `ignore` as a readonly tuple, which doesn't match
`UserConfig.ignore: string[]`, breaking CI typecheck.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* test: integrate code-block copy regression into existing copy test suite

Address PR review:
- Drop standalone codeBlockMarkdown.test.ts; add `codeBlockFullContent` and
  `codeBlockPartialSelection` cases to copyTestInstances.ts and snapshot
  text/plain markdown for all copy test instances via a new
  `Copy tests (Markdown)` describe block (mirrors the export test pattern).
- Trim verbose comments in copyExtension and serializeBlocksExternalHTML.
- copyPasteEquality executor now passes the actual markdown payload as the
  text/plain MIME instead of a literal "text" placeholder, so paste handlers
  that prefer text/plain (e.g. inside code blocks) round-trip correctly.
- Update mixedInParagraph snapshots: prettify's `ignore: ["code"]` preserves
  trailing whitespace inside `<code>` spans, which is the actual HTML output.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>