fix(chat): drop the streaming cursor that detached onto its own line

[BunsDev]

Reported from the running app: a small grey bar sitting on its own line between
a message and whatever followed it.

What it was

The streaming cursor (▌) rendered as a sibling of the markdown container:

<div className="cave-md" dangerouslySetInnerHTML={{ __html: html }} />
{pending ? <span …>▌</span> : null}

That placement was chosen deliberately — it is the only way to show a caret
without injecting into the sanitized HTML string — but .cave-md is a
block-level <div>, so the span can never sit at the end of the last rendered
line. It necessarily wraps onto a row of its own.

So this was not specific to one message. It appeared after every streaming
message that had rendered markdown, and read as a rendering artifact rather
than as a caret.

Why remove rather than reposition

Putting a caret back at the end of the text means appending inside the last
inline descendant of the sanitized HTML — exactly the injection the sibling
placement existed to avoid. Progress is already legible from the text growing,
so the affordance goes rather than gets moved.

The plain-text fallback cursor goes with it. It only shows for the window
before the first render lands, so keeping it would flash a caret for a moment
and then lose it for the rest of the stream — a flicker, not an affordance.

pending still drives progressive rendering; only the glyph is gone.

Tests

Two source-text assertions previously required the cursor and would have
failed this change. They now assert its absence at both sites, so it cannot
creep back without the block-container problem being solved first. A third
assertion in session-debug.test.ts still pins that only the last text span
streams — that behaviour is unchanged; only its wording mentioned the glyph.

Verified: pnpm test:app (1161 files), pnpm lint, pnpm typecheck.

Closes cave-1yslk.

[Copilot]

Pull request overview

Removes the streaming cursor glyph from MessageBubble markdown/plain-text rendering to eliminate a detached “grey bar” artifact caused by rendering the cursor as a sibling of a block-level markdown container.

Changes:

• Remove the pending-gated cursor glyph from both the plain-text fallback and sanitized-HTML markdown render paths in MarkdownContent.
• Update markdown-source tests to assert the cursor glyph is absent and prevent reintroduction without solving the layout constraint.
• Update session-debug assertions/comments to reflect that progressive rendering remains, but the cursor glyph is gone.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/components/message-bubble.tsx	Removes the cursor glyph rendering in MarkdownContent; adds explanatory inline comment about why it’s removed.
src/components/message-bubble-markdown.test.ts	Updates source-text assertions to enforce absence of the cursor glyph and sibling pending-gated cursor span.
src/lib/session-debug.test.ts	Updates pinned description/assertion text to remove cursor-glyph wording while keeping progressive-render behavior checks.

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