test(markdown): stabilize issue SSR mocks

[beruro]

Problem

The current develop branch fails four synchronous SSR tests across the shared GitHub issue surface. Those tests render through the lazy Markdown entry without a synchronous test substitute, causing React to suspend inside renderToStaticMarkup. Once that suspension is removed, the same stale test mocks also omit the RICH_MARKDOWN_COMPOSER_TOOLBAR_CLASS export now consumed by the issue comment composer. The production UI has a Suspense host; the regression is in the test boundary and blocks frontend CI for unrelated PRs.

Solution

Update the two affected SSR test modules to provide:

• a synchronous MarkDown substitute that still renders the supplied text content; and
• the public toolbar-class constant alongside the existing RichMarkdownEditor substitute.

The tests continue rendering the real issue/thread composition and exercising their original assertions; no production component or behavior changes.

Potential risks

The synchronous Markdown substitute does not exercise lazy chunk loading or Markdown parsing in these two composition tests. Those concerns belong to the Markdown component's own tests; these tests verify issue metadata, controls, timeline composition, and shared surface structure. The patch changes only test mocks, with no UI, persistence, API, dependency, or runtime impact. Rollback is a normal revert of this commit.

Verification

• Reproduced on unmodified develop: the two target files reported 4 failures / 5 passes due to synchronous suspension.
• PATH=../../../node_modules/.bin:$PATH vitest run src/modules/ProjectManager/WorkItems/components/GitHubIssueThreadSurface.test.ts src/modules/WorkStation/CodeEditor/Panels/EditorPrimarySidebar/content/IssuesContent/__tests__/IssueDetailExternalLinkButton.test.ts — passed, 9/9 tests after the fix.
• PATH=../../../node_modules/.bin:$PATH tsc --noEmit — passed.
• Prettier and git diff --check — passed.
• Final scope inspection — two test files, 16 added lines, no production changes, personal paths, secrets, artifacts, or debug output.
• GitHub CI is queued on the published head. Current develop still has the independent imported-history Rust Clippy baseline failure isolated in fix(history): satisfy imported window clippy #752; this test-only PR intentionally does not absorb that unrelated production fix. After either baseline PR lands, the remaining PR must be rebased and rerun.
• UI screenshots were not captured because this is a test-boundary repair with no rendered product change.

[beruro]

Merge-order note: this PR fixes the current develop frontend baseline; #752 independently fixes the current develop Rust Clippy baseline. Keeping them separate preserves the repository single-responsibility contract. If branch protection requires both jobs green, a maintainer must land either one with the unrelated baseline failure acknowledged, then rebase and rerun the other; subsequent PR CI can then be rerun on a clean base.

[Harry19081]

develop already contains the same fix in commit d3ffa4376, merged through PR #749.