feat(asides): attach standalone asides to preceding paragraphs

[cpsievert]

Aside markers now stay beside the paragraph they support even when generated Markdown contains a blank line before the aside. This removes formatting sensitivity for server-verified citations while preserving normal block boundaries.

Motivated by posit-dev/commons#127

What changes

• Attach an aside-only paragraph or direct aside sibling to the immediately preceding paragraph under the same parent.
• Resolve attachment before grounded-text matching, grouping, and streaming state.
• Preserve aside order, rich bodies, and list-item ownership.
• Leave asides unchanged across code, lists, tables, headings, quotations, and mixed-content paragraphs.
• Rebuild and distribute the browser assets for Python and R.

Verification

• npm test — 1,038 passed, 23 skipped
• npm run lint
• npm run build

[Copilot]

Pull request overview

This PR improves Markdown/HTML aside handling so standalone <shiny-aside> blocks (including those separated by a blank line in generated Markdown) are automatically attached to the immediately preceding paragraph under the same parent, enabling correct grounding/grouping without being sensitive to formatting.

Changes:

• Added rehypeAttachAsidesToPreviousParagraph and integrated it into both the Markdown and HTML processing pipelines ahead of grounding/grouping.
• Simplified rehypeGroupAsides by removing list-item-specific “move asides into paragraphs” logic now covered by the new attachment plugin.
• Expanded test coverage across processor integration, the new plugin’s behavior, trailing-aside streaming behavior, and React rendering.

Reviewed changes

Copilot reviewed 10 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg-r/inst/lib/shiny/GIT_VERSION	Updates packaged asset version reference for the R distribution.
pkg-py/src/shinychat/www/GIT_VERSION	Updates packaged asset version reference for the Python distribution.
js/tests/markdown/processors.asides.test.ts	Adds end-to-end tests asserting correct grounding/numbering when asides are separated by blank lines or are direct HTML siblings.
js/tests/markdown/plugins/rehypeGroupAsides.test.ts	Updates grouping tests to reflect new attachment behavior and verify <br> trimming before grouped asides.
js/tests/markdown/plugins/rehypeAttachAsidesToPreviousParagraph.test.ts	Adds focused unit tests for the new attachment plugin across paragraphs, raw HTML siblings, and list-item ownership boundaries.
js/tests/markdown/plugins/markTrailingAsides.test.ts	Ensures trailing-aside “pending” marking works after automatic attachment (streaming path).
js/tests/markdown/MarkdownContent.test.tsx	Validates React rendering attaches direct HTML asides to the preceding paragraph.
js/src/markdown/processors.ts	Wires the new attachment plugin into both markdownProcessor and htmlProcessor in the intended order.
js/src/markdown/plugins/rehypeGroupAsides.ts	Removes now-redundant loose-list-item aside relocation logic.
js/src/markdown/plugins/rehypeAttachAsidesToPreviousParagraph.ts	Introduces the new rehype plugin that attaches standalone asides to the previous paragraph under the same parent.

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