Fix unreachable!() panic when DOCTYPE appears between text runs in element content

[williamareynolds]

Summary

The serde deserializer panics with internal error: entered unreachable code (src/de/mod.rs read_text's "Cannot be two consequent Text events" branch) when a DOCTYPE declaration appears between text runs inside element content.
Minimal repro:

use serde::Deserialize;

#[derive(Deserialize)]
struct Wrapper { #[serde(rename = "$text")] text: String }

// Panics on master:
//   thread '...' panicked at quick-xml/src/de/mod.rs:2910:21:
//   internal error: entered unreachable code
let _: Wrapper = quick_xml::de::from_str(r#"<a>x<!DOCTYPE y>z</a>"#).unwrap();

I found this while fuzzing a new library I'm working on. I'm not an expert on quick-xml so I hope I got this all right.

Root cause

Deserializer::drain_text is the merge step that guarantees read_text (and friends) see at most one DeEvent::Text per element. It loops while current_event_is_last_text returns false, draining Text / CData / GeneralRef
payload events into a single result.

DocType is not in current_event_is_last_text's match arm, so the loop exits when a DocType appears. The outer Deserializer::next() then:

1. Returns the accumulated DeEvent::Text for the first text run.
2. On the next call, processes the DocType (captures it via the entity resolver) and continues.
3. Returns the second text run as a separate DeEvent::Text.

read_text, expecting a single text run followed by End, hits the consecutive-Text unreachable!().

Fix

Treat DocType as transparent during the text drain:

• current_event_is_last_text now also returns false when the lookahead is DocType, so drain_text keeps draining instead of breaking at a DOCTYPE boundary.
• drain_text's match arm gains a PayloadEvent::DocType(e) case that forwards the event to entity_resolver.capture() (identical to the existing Deserializer::next() path) and continues draining.

DTD entities defined inside the document body remain available for subsequent &entity; resolution — the entity resolver sees DOCTYPE events in exactly the same order as before. The only observable change is that the surrounding text
runs merge into one DeEvent::Text, which matches what read_text and the rest of the serde deserializer have always assumed.

Tests

Adds a new doctype_in_element_text module in tests/serde-de.rs with three cases:

Case	Input
single_doctype_between_text	<a>x<!DOCTYPE y>z</a>
multiple_doctypes_between_text	<a>x<!DOCTYPE y><!DOCTYPE z>w</a> (matches the fuzzer-discovered shape)
leading_doctype_then_text	<a><!DOCTYPE y>x</a>

All three previously panicked, now pass. Full cargo test --all-features stays green (1,711 passing, no regressions).

MSRV / fmt / minimal-versions

• cargo fmt --check: clean.
• cargo check on rust-toolchain 1.79.0: clean (no new language or stdlib features used).
• No new dependencies; the fix is local to src/de/mod.rs plus the new test cases.

[Mingun]

Generally approve except needing to check the trailing whitespace handling question.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 28.57143% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.31%. Comparing base (a759d65) to head (e00ae5c).
⚠️ Report is 54 commits behind head on master.

Files with missing lines	Patch %	Lines
src/de/mod.rs	28.57%	10 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #964      +/-   ##
==========================================
+ Coverage   55.08%   57.31%   +2.22%     
==========================================
  Files          44       46       +2     
  Lines       16911    18197    +1286     
==========================================
+ Hits         9316    10429    +1113     
- Misses       7595     7768     +173     

Flag	Coverage Δ
unittests	57.31% <28.57%> (+2.22%)	⬆️

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