Improve code actions

[renkun-ken]

Summary

• Add preferred quick fixes for common lintr diagnostics.
• Add conflict-aware source.fixAll support and advertise the supported code-action kinds.
• Apply grouped nolint actions across all affected lines.
• Correct the code-action request parameter shape.

Testing

• Focused code-action unit and integration tests pass.
• Generated fix-all edits clear real lintr diagnostics in a mixed sample.
• The branch diff passes the Git whitespace check.