[ty] Preserve blank lines between comments and imports in add-import action

[mvanhorn]

Summary

Fixes astral-sh/ty#2990

When the add-import code action inserts a new import after a comment block (e.g., PEP 723 script metadata), it dropped the blank line between the comment and the existing imports. After a formatter runs, this caused the comment to visually merge with the import block.

Before (blank line lost after inserting from anyio import AsyncFile):

# /// script
# dependencies = ["anyio"]
# ///
from anyio import AsyncFile

import datetime as dt

After this fix (blank line preserved):

# /// script
# dependencies = ["anyio"]
# ///

from anyio import AsyncFile
import datetime as dt

The fix tracks blank lines encountered after the last comment in the start_of_file insertion logic. When blank lines exist between the final comment and the first statement, the insertion point advances past them to preserve the gap. Blank lines that appear before any comments are not affected.

Test plan

• Added two test cases to start_of_file tests: one with a blank line between comments and imports (verifying the gap is preserved), and one without (verifying no regression)
• All 293 ty_ide import tests pass
• All 3 ruff_python_importer tests pass
• cargo clippy and cargo fmt clean

[MichaReiser]

Thank you

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[mvanhorn]

Thanks for the merge!