Five fixes for non-idempotent formatting where running the formatter
twice produces different output:

1. Long if/else attribute values inlined unconditionally.
   write_attribute_if_chain always wrote if cond { val } else { val }
   on one line. For long values (e.g. Tailwind classes) this exceeds 80
   chars, causing ShortOptimization to oscillate. Now checks inline
   length against a budget and uses a deterministic multiline layout:

   class:
       if active {
           "long-class-string"
       } else {
           "another-long-class-string"
       },

2. Blank lines in empty component bodies.
   write_todo_body emitted a newline even with no comments between
   braces. Now only emits when actual // comments are found.

3. split_line_attributes override on empty blocks.
   The override forced NoOpt even for Empty blocks (no attrs,
   children, or spreads), causing Foo {} to expand with blank lines.
   Now skips the override when the block is Empty.

4. Spurious blank lines from trailing comments logic.
   accumulate_full_line_comments collected empty lines even when no
   // comments existed. The trailing comment paths in write_rsx_block
   and write_trailing_body_comments now check for real comments before
   emitting.

5. Spurious blank line in NoOpt path for children-only blocks.
   Elements with children but no attributes (e.g. li { button { ... } })
   got a blank line after the opening brace because NoOpt emitted
   new_line() for attributes then another new_line() before children.
   Now skips the extra newline when there are no attributes or spreads.

Fixes #5508

Test plan

• long_if_else_attr.rsx: twoway + idempotency
• empty_component_body.rsx: twoway + idempotency
• empty_braces_oneliner.rsx: twoway + 3-pass idempotency
• empty_braces_match_arm.rsx: 3-pass idempotency
• All 90 existing + new tests pass
• Tested on a ~4K LOC Dioxus codebase with split_line_attributes = true:
  idempotent across all files after formatting