Post list preview: trim trailing newline in generated excerpt

[YuF-9468]

Description

Fixes #12673.

This updates post preview generation to trim leading/trailing whitespace and newlines after extracting/collapsing preview text. It prevents preview labels from rendering a trailing blank line in the post list when content contains paragraph separators.

Testing instructions

1. Create or edit a post with multiple paragraph blocks.
2. Return to the post list.
3. Verify the preview no longer shows an extra blank trailing line/empty space under the first visible line.

Automated verification

• Added testThatContentPreviewForDisplayTrimsLeadingAndTrailingNewlines in PostTests.
• CI should run the updated test suite for this PR.

[jkmassel]

Thanks for taking a look at this! The implementation looks good, I just left a note about improving one test.

I appreciate the contribution, and I'm excited to get it merged!

[YuF-9468]

Great call — updated ✅

I changed the test to assert the explicit expected output instead of mirroring implementation details:

• XCTAssertEqual("Paragraph 1\nParagraph 2", preview)

Commit: 9f5831e

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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