Preserve newlines in notification messages

[maruthang]

Summary

NotificationViewItem.parseMessage previously flattened all line breaks to spaces:

// Before
message = message.replace(/(\r\n|\n|\r)/gm, ' ').trim();

This PR keeps \n intact and normalizes \r\n / \r to \n. The renderer (NotificationMessageRenderer.render) splits plain string nodes on \n and emits a <br> between segments, so multi-line notification messages render with the line breaks the producer intended.

Fixes #178796

Implementation

• src/vs/workbench/common/notifications.ts — change the parse-time strip to a normalize-to-\n. Link parsing still works because the link pattern in parseLinkedText does not span lines, so any \n always ends up in a plain string node before/after a parsed link.
• src/vs/workbench/browser/parts/notifications/notificationsViewer.ts — when iterating message.linkedText.nodes, split each plain string on \n and emit a <br> element between segments. Empty segments are skipped so a trimmed-trailing-newline message doesn't append a stray <br>.

No CSS changes needed

• Expanded notification messages already declare white-space: normal; word-wrap: break-word; and the offset helper used for auto-expansion height calculation already wraps.
• Collapsed rows have a fixed LINE_HEIGHT (22) with overflow: hidden, so the single-line preview look is preserved while the offset helper correctly computes a taller preferred height — driving auto-expansion already works.

Accessibility

notificationsAlerts.ts reads notification.message.linkedText.toString() for ARIA alerts. Screen readers naturally pause at \n, so multi-line announcements read more naturally than today's space-flattened output.

message.raw is unchanged — it's set before the parse-time normalization, so existing aria-label/clipboard-copy callers (notificationsToasts.ts, notificationsList.ts, notificationsActions.ts) see no behavior difference.

Conflict note for reviewers

This PR conflicts with my open PR #313707 (Render inline code spans in notification messages, fixes #191713) on the same two files. Both touch the notifications.ts parse path and the notificationsViewer.ts rendering. The conflict is mechanical — one introduces a nodes: NotificationMessageNode[] shape; this one preserves \n in plain text nodes. Whichever lands first, I'm happy to rebase the other.

Test plan

• New regression test in notifications.test.ts covering: a plain \n is preserved through parsing; \r\n and \r are normalized to \n; leading/trailing newlines are trimmed; newlines coexist with markdown links.
• Manual: post a notification with \n in the message via vscode.window.showInformationMessage("Line one\nLine two") from an extension or notification.show({ severity: Severity.Info, message: "Line one\nLine two" }) — verify both lines render.
• Manual: ensure existing single-line notifications render identically (no extra <br>).
• Manual: a multi-line message with a link mid-message keeps both the line breaks and the parsed link.

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