Fix StyledText crash with dynamic custom_highlights

[KlausUllrich]

Problem

StyledText::with_runs panics with "invalid text run" when custom_highlights are used with dynamically updating content via TextView::markdown() or TextView::html().

This affects any use case where:

• Content updates frequently (log viewers, live data displays, streaming text)
• with_highlights() is used to apply custom styling
• The panic occurs at gpui/src/elements/text.rs:251

Root Cause

Two issues in text/node.rs:

1. NodeContext equality ignores custom_highlights

The PartialEq impl for NodeContext intentionally excludes custom_highlights from comparison. When highlights change but text content doesn't, GPUI's diffing skips re-rendering and reuses stale Inline elements with mismatched TextRun indices.

2. merge_custom_highlights missing bounds validation

merge_custom_highlights() clips highlight ranges to chunk boundaries but doesn't validate the result. When chunks from parsed markdown and highlights from the caller originate from different parse cycles, clipping can produce local_end > chunk_len, leading to the panic downstream.

Fix

1. Include custom_highlights in NodeContext::PartialEq so highlight changes trigger re-rendering
2. Add bounds validation in merge_custom_highlights() to skip highlights that exceed chunk bounds after clipping

Both changes are minimal and defensive. The equality fix ensures correctness; the bounds check adds safety for edge cases during content transitions.

Testing

Reproduced in a production log viewer with rapid content updates and custom syntax highlighting. Both fixes together eliminate the crash class.