fix(markup): restore code copy newline strip and fix copying rendered blocks (#39755)

1. Fix JS error when copying Mermaid and Math block sources, regression
from https://github.com/go-gitea/gitea/pull/39718.
1. Restore the newline trim which is consistent with the majority of
forges and markdown renderers as per
https://github.com/go-gitea/gitea/pull/39751#issuecomment-6129002625 and
my strong preference.

This is the minimal fixes to fix the bug and restore the existing
behaviour, more discussions and followups can be had in
https://github.com/go-gitea/gitea/pull/39751.

tests/e2e/mermaid.test.ts

@@ -13,5 +13,9 @@ test('mermaid diagram in issue', async ({page, request}) => {
   const svg = page.frameLocator('iframe.markup-content-iframe').locator('svg');
   await expect(svg).toContainText(/Alpha[\s\S]*Beta[\s\S]*Gamma/);
 
+  await svg.hover();
+  await page.locator('.code-copy').click();
+  await expect.poll(() => page.evaluate(() => navigator.clipboard.readText())).toBe('flowchart LR\n  Alpha --> Beta\n  Beta --> Gamma');
+
   await assertNoJsError(page);
 });