Fix MarkupError crash when hovering a Markdown table cell with bracketed text

[jryyangjy]

Problem

MarkdownTableContent.compose() sets a table cell's tooltip from cell.plain -- a plain str extracted from the cell's Content:

yield MarkdownTableCellContents(
    cell,
    classes=f"row{row_index} cell",
    name=f"cell{row_index}.{cell_index}",
).with_tooltip(cell.plain)

Tooltips set from a str are rendered with markup enabled (Static.update() -> visualize(..., markup=self._render_markup) -> Content.from_markup(obj), see textual/visual.py). If the cell's text happens to contain something that looks like markup -- e.g. ([sys:LOCK=SAFE…]) -- hovering the cell raises MarkupError and crashes the app.

The streaming path (_update_rows, used when a Markdown widget is fed incrementally via get_stream) already avoids this by passing the Content object directly: .with_tooltip(cell). Because Content already implements the Visual protocol, visualize() returns it unchanged and never re-parses it as markup. Header cell tooltips (.with_tooltip(header), a few lines above) already do this correctly too -- only the row-cell path in compose() had the bug.

Verification

Reproduced the crash headlessly with run_test() (no real terminal needed), using the exact repro from the issue:

HEADER = "| session | note |
| --- | --- |
"
SAFE_ROW = "| aaa-bbb-ccc | first row padding to force truncation |
"
BRACKET_ROW = (
    "| 019f314b-d61d-7837-a158-8aac342ae56f "
    "| ([sys:LOCK=SAFE…]) persona override |
"
)

• On main: the bracketed cell's tooltip is a plain str, and rendering it with markup enabled raises textual.markup.MarkupError: Expected markup value (found '…]) persona override').
• After the fix: the tooltip is the Content object, and rendering it raises nothing.
• Confirmed with a real editable install of this branch in an isolated venv (not just source inspection) -- ran the assertion both ways by stashing/restoring the source change to make sure the added test actually fails without the fix and passes with it.

Fix

• _markdown.py: pass cell instead of cell.plain to with_tooltip() in the one-shot compose() path, matching the existing streaming path and the header-tooltip path.
• Added a regression test (test_markdown_table_cell_tooltip_with_bracketed_text_does_not_crash) that mounts a one-shot markdown table with bracketed cell text and asserts every cell's tooltip renders (via visualize(..., markup=True)) without raising.
• Added a CHANGELOG entry under ## Unreleased, following this repo's existing pattern for in-progress fixes.

Testing

• pytest tests/test_markdown.py: 26 passed (including the new regression test).
• pytest tests/snapshot_tests/test_snapshots.py -k markdown: 10 passed, 1 pre-existing failure (test_text_area_language_rendering[markdown], a TextArea syntax-highlighting snapshot unrelated to this change -- confirmed it fails identically on unmodified main, likely a local font/terminal rendering mismatch against the reference snapshot).

Fixes #6642