Draft: Persist rich tool displays with htmltools

[cpsievert]

Why this matters

The immediate serialization fix deliberately keeps shinychat's existing dependency dictionaries so it can ship without waiting for another package release. Those dictionaries are suitable for the browser, but they omit the dependency source needed to register local CSS and JavaScript in a later session.

With the RenderedHTML value object proposed in posit-dev/py-htmltools#123, the result of render() owns its source-preserving durable representation. Shinychat can use that public boundary instead of maintaining a parallel dependency schema.

What changes

• Delegate new rich-display writes to htmltools.serialize_html(), the convenience API over RenderedHTML.to_serialized().
• Restore new values with htmltools.deserialize_html().
• Keep a small reader for dependency dictionaries written by older shinychat versions.
• Remove session behavior from deserialization; Shiny still registers and rewrites dependencies when UI is processed for the browser.
• Construct RenderedHTML through its public constructor so the code works with both the current TypedDict and the proposed value object.

Compatibility

Old records remain readable. Their HTML and previously resolved dependency URLs are preserved, but local sources cannot be recovered because older shinychat versions did not store them. Once an old value is loaded and written again, it uses the new schema.

Verification

• uv run --no-sync pytest -q --ignore=pkg-py/tests/playwright (431 passed)
• The fresh-app regression verifies that a restored local dependency mounts at /lib/my-dep-1.0.
• uv run --no-sync ruff check pkg-py --config pyproject.toml
• uv run --no-sync pyright
• Source distribution and wheel build

The repository-wide Ruff format check still reports the inherited formatting difference in pkg-py/tests/playwright/chat/aside_responsive/test_aside_responsive.py.

Before marking ready

• Merge and release Let rendered HTML own source-preserving serialization py-htmltools#123 as htmltools 0.8.0.
• Refresh uv.lock and verify uv sync --all-extras from the package index.
• Rebase this PR onto shinychat main after Fix Python tool-result display serialization #297 and Refine Python tool-result display serialization #315 merge.