Fix markdown_parser: unclosed <u> markers no longer delete surrounding text

[arpansahu]

Description

parse_markdown was silently deleting text when a <u> underline marker was left unclosed and another <u> appeared later in the same line.

<u> is tokenized as an UnderlineStart delimiter and pushed onto the shared delimiter stack. process_emphasis treats two <u> markers as a matchable opener/closer pair (since can_open_for only checked kind equality), consuming both markers and removing the text nodes between them.

Underline is only ever supposed to be closed by an explicit </u>, which is handled separately by parse_underline (it never goes through can_open_for). Pairing two <u> openers through the emphasis path is therefore always wrong.

Fix

can_open_for now explicitly rejects DelimiterKind::UnderlineStart openers, so an unclosed <u> round-trips to literal text — the same behavior as an unclosed * or ~~.

Input	Before	After
<u><u>	empty line — all text gone	<u><u>
<u>a<u>	a (both <u> deleted)	<u>a<u>
a <u>word<u> b	a word b (both <u> deleted)	a <u>word<u> b

Linked Issue

Fixes #12863

• The linked issue is labeled ready-to-implement.
• No UI changes — this is a pure parser logic fix in crates/markdown_parser.

Testing

• Added test_parse_unclosed_underline_round_trips_to_literal_text, covering the three repro cases from the issue (<u><u>, <u>a<u>, a <u>word<u> b).
• Ran cargo test -p markdown_parser: all 155 tests pass (154 pre-existing + 1 new).
• Ran cargo fmt -p markdown_parser --check: clean.
• Ran cargo clippy -p markdown_parser --all-targets --all-features -- -D warnings: no warnings.

This is a crate-scoped, platform-independent parser fix with no UI surface, so it was verified via the automated test suite above rather than manual app testing (./script/run builds the full desktop app, which isn't feasible in this environment).

Screenshots / Videos

N/A — parser-only change, no UI.

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@arpansahu

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Overview

This PR updates markdown_parser so bare <u> start tags are not matched as opener/closer pairs by the emphasis algorithm; underline remains closed only by explicit </u>. It also adds focused unit coverage for the reported unclosed-underline cases.

Concerns

• None found.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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