don't intercept pastes inside citation ids

[kevinushey]

Addresses rstudio/rstudio#18295

Problem

In the visual editor on Windows, typing @ and pasting a DOI (e.g. https://doi.org/10.1000/182) does nothing, whereas on Linux/macOS the editor offers to look up the DOI and insert a citation.

Cause

ProseMirror consults handlePaste plugins in registration order, and the generic paste extension (behaviors/paste.ts) is registered before the cite mark's paste_cite_doi plugin. Inside a cite_id mark the generic handlers form a pincer that prevents paste_cite_doi from ever running on Windows:

• Clipboards carrying text/html (e.g. a DOI copied from a browser page) are consumed by pasteHtmlHandler's isWindows() branch (the workaround for ProseMirror freezing on multi-paragraph pastes).
• Plain-text-only clipboards are consumed by pasteMarkdownHandler (the cite marks don't set noInputRules, so allowMarkdownPaste returns true inside a citation). This variant also affects macOS and Linux.

Since isWindows() keys off the user agent, this affects any Windows client, including browsers connecting to a Linux server.

Fix

Both generic handlers now return false when the selection is inside a cite_id mark, allowing the cite mark's own paste handler to process the paste (insert the DOI, offer to insert a citation, or fall back to a plain-text insert for non-DOI content).

Tests

Added apps/vscode/src/test/editor-paste.test.ts covering the fixed cases plus the preserved behaviors (Office content and Windows text/html pastes outside citations are still handled, plain text outside citations is still pasted as markdown). The 3 regression cases fail without the fix.

The tests live in the vscode extension suite (rather than packages/editor, whose jest-style tests under test/api are orphaned -- no runner is configured for them, and test/README.md notes their fixtures no longer work) because it is the only harness exercised by CI, and this avoids adding a new test runner dependency. The tests exercise the editor package directly and don't use the vscode API; the only dependency change is adding the editor workspace package as a devDependency of the extension.

Verified tsc --noEmit and the panmirror bundle build pass. Manual verification on Windows is still pending.

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