fix(ui): preserve indentation in diff-backed tool rendering

[srothgan]

Summary

• preserve leading source indentation in diff-backed tool rendering while keeping the 0.11.2 line-number and hanging-indent layout
• keep nested diff indentation inside non-Execute tool cards and align capped Write omission markers with the rest of the diff block
• add regression tests for raw diff indentation, wrapped diff continuations, and tool-card diff rendering

Why

The 0.11.2 diff renderer improved tool-call diffs with line numbers and hanging indents, but diff rows still dropped leading source whitespace during wrapping. That made indented code inside Edit and Write cards render flush-left in the TUI, which looked wrong and made nested code structure harder to read.

Closes #

Validation

• Automated:
  • cargo fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test
  • cargo check --offline
• Manual:
  • @srothgan confirmed the updated TUI rendering now looks correct for indented diff content

Notes

• Breaking changes: N/A
• Docs updated: N/A