Preserve selected message in inspector on updates

[adranjan4]

Summary

• Fixes inspector overrides user's selected message on update #247: inspector no longer steals selection when new LSP messages arrive
• Auto-follows the latest row only when the cursor is already on the last message
• Applies to both lsp-devtools inspect and the client inspector

Test plan

• cd lib/lsp-devtools && hatch test (or pytest on tests/inspector/test_message_browser.py)
• Run lsp-devtools inspect, feed chatty traffic, select a middle row — selection should stick
• Move to the last row — new messages should still follow the tail

[adranjan4]

pre-commit.ci is failing with the same ruff/mypy issues present on develop (not introduced here). Happy to help clean those up in a follow-up if needed.