editor: Reveal search match after manual scroll

[hlcfan]

Description

Fix Next/Previous advancing the search counter without revealing the match after manual scrolling. Search navigation now scrolls in either direction while preserving configured surrounding-line padding; cursor navigation keeps its existing direction restrictions.

Validation

• Added regression tests for Next scrolling upward and Previous scrolling downward, including surrounding-line padding.
• cargo test -p gpui-base input:: --lib --offline — 152 tests passed locally.

AI assistance: Codex generated the implementation and regression tests.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific) — Not applicable; the search navigation fix is platform-independent.