console: fix search in the flow view - #8355

Description

Searching inside a flow (/, then n/N) is broken in mitmproxy 12: on a match the whole message body turns grey, loses its syntax highlighting, and n/N do not move anywhere.

FlowDetails._get_content_view renders the entire body as a single widget (text_objects = [urwid.Text(chunks)]); before the contentview rework (#7623, #7670) there was one widget per line. Since Searchable works on list walker items, this breaks search in three ways:

1. Highlight rebuilds the item as AttrMap(urwid.Text(t.text), "focusfield") — plain text without markup, so syntax highlighting is discarded, and focusfield is ("black", "light gray"), repainting the whole body grey.
2. find_next iterates over items, not over occurrences within an item, so with a single body item n/N never advance.
3. last_search lives on the Searchable instance, but conn_text() creates a new one on every flow view rebuild, so the search term is silently forgotten.

Changes:

• flowview.py: add chunks_to_lines() to split highlighting chunks at newlines, so bodies are rendered one widget per line again. Applied to HTTP bodies, TCP/UDP streams and WebSocket messages.
• searchable.py: Highlight keeps the original markup and emphasizes only the matched substrings.
• searchable.py: move the last search term into a shared SearchState so n/N survive view rebuilds.

test/mitmproxy/tools/console/test_flowview_search.py covers all three symptoms; the tests fail on current main and pass with this change.

Checklist

• I have updated tests where applicable.
• I have added an entry to the CHANGELOG.