TypeError: Cannot read properties of undefined (reading 'match') when a commit subject contains a raw `<details>` token - crashes re-parsing of the open release pull request body

### Environment details

- `release-please` **17.6.0** via `googleapis/release-please-action@v5`, manifest mode, single package (`node` strategy)
- Also verified against **17.11.1** (latest on npm today): still affected
- Related: #1659 (fixed by #1661) and #2801 (regression of the former, still open). This report documents a second, harder failure mode of the same underlying gap: raw HTML-looking tokens from commit subjects reach the PR body and are later re-parsed by an HTML parser.

### Steps to reproduce

1. Land a commit whose conventional-commit subject contains an inline-code token that is itself an HTML tag name, e.g.

```
git commit -m 'fix: escape an unbalanced `` tag instead of refusing the draft'
```

2. Let release-please update the open release pull request - the changelog bullet now contains a literal `` string inside backticks.
3. On the **next** push to the default branch, the workflow fails every time until the pull request is merged or its body is edited by hand:

```
✔ Looking for open release pull requests
##[error]release-please failed: Cannot read properties of undefined (reading 'match')
```

Full stack trace (captured by invoking `Manifest.createPullRequests()` directly against the affected repository):

```
TypeError: Cannot read properties of undefined (reading 'match')
at extractMultipleReleases (build/src/util/pull-request-body.js:104)
at PullRequestBody.parse (build/src/util/pull-request-body.js:38)
at FilePullRequestOverflowHandler.parseOverflow (build/src/util/pull-request-overflow-handler.js:74)
at Manifest.findOpenReleasePullRequests (build/src/manifest.js:512)
```

### Root cause

1. `htmlEscape()` in `src/changelog-notes/default.ts` deliberately leaves `<`/`>` untouched inside inline code spans (so markdown rendering stays correct).
2. The generated release PR body therefore contains a literal `` `` `` string.
3. On every subsequent run, `Manifest.findOpenReleasePullRequests()` re-parses that body. `extractMultipleReleases()` in `src/util/pull-request-body.ts` uses `node-html-parser`, which does not understand markdown: the backticked token becomes a real `` element **without a `` child**.
4. The code then dereferences it unconditionally:

```ts
const summaryNode = detail.getElementsByTagName('summary')[0];
const summary = summaryNode?.textContent;
const match = summary.match(SUMMARY_PATTERN); // <- throws when summaryNode is missing
```

Note the asymmetry: `#2801` shows a raw token like `` silently swallowing subsequent `` blocks (component skipped); a raw token that *is* `` produces a summary-less element and this unguarded `.match()` turns the same gap into a hard crash that blocks all releases.

### Minimal reproduction

```js
const {PullRequestBody} = require('release-please/build/src/util/pull-request-body');
const body = [
':robot: I have created a release *beep* *boop*',
'---',
'',
'## [0.9.1](https://github.com/example/repo/compare/v0.9.0...v0.9.1) (2026-08-22)',
'',
'### Bug Fixes',
'',
'* escape an unbalanced `` tag instead of refusing the draft ([#133](https://github.com/example/repo/issues/133))',
'',
'---',
'footer',
].join('\n');
console.log(PullRequestBody.parse(body));
// => TypeError: Cannot read properties of undefined (reading 'match')
// Same body with `` replaced by e.g. `<details>` parses fine.
```

### Suggested fix

Skip `` blocks that have no `` (warn and continue), so parsing falls back to the single-release path instead of crashing. Real generated blocks always carry a ``, so nothing else changes. I have a patch with tests ready and will open a PR referencing this issue.

Start in src/util/pull-request-body.ts, especially extractMultipleReleases and PullRequestBody.parse, and reproduce the failure with the minimal body shown in the issue. Done means a details block without a summary is skipped with a warning, parsing falls back to the single-release path, and the raw details token no longer crashes release PR discovery; add or run the related parser tests.

Written by the indexing model from the issue text.