Add @do annotation coverage tests

[proboscis]

Summary

• add tests/public_api/test_do_annotation_coverage.py
• cover the 11 requested @do auto-unwrap annotation variations end-to-end through the real runtime pipeline
• exercise from __future__ import annotations resolution by keeping the whole test module on string annotations and asserting the raw annotation stays a string

Acceptance Criteria Evidence

1. One test file with all variations

• Added tests/public_api/test_do_annotation_coverage.py

2. Annotated ProgramLike/Effect/DoExpr args are not auto-unwrapped

• Covered by test_program_varargs_annotation_preserves_program_objects
• Covered by test_doexpr_varargs_annotation_preserves_program_objects
• Covered by test_programlike_varargs_annotation_preserves_program_and_effect_objects
• Covered by test_program_union_with_none_annotation_preserves_program_object
• Covered by test_annotated_program_annotation_preserves_program_object
• Covered by test_effect_varargs_annotation_preserves_effect_objects
• Covered by test_effect_subclass_annotation_preserves_custom_effect_object
• Covered by test_future_annotations_are_resolved_through_the_do_pipeline

3. Baseline tests verify unannotated/Any/str varargs do auto-unwrap

• Covered by test_unannotated_varargs_auto_unwrap_programlike_values
• Covered by test_any_varargs_auto_unwrap_programlike_values
• Covered by test_str_varargs_auto_unwrap_programlike_values

4. Tests were run through the actual @do pipeline

$ uv run pytest tests/public_api/test_do_annotation_coverage.py
collected 11 items
tests/public_api/test_do_annotation_coverage.py ...........              [100%]
============================== 11 passed in 0.04s ==============================

$ uv run pytest tests/public_api/test_types_001_kpc.py tests/public_api/test_varargs_program_no_unwrap.py tests/public_api/test_do_annotation_coverage.py
collected 40 items
tests/public_api/test_types_001_kpc.py .........................         [ 62%]
tests/public_api/test_varargs_program_no_unwrap.py ....                  [ 72%]
tests/public_api/test_do_annotation_coverage.py ...........              [100%]
============================== 40 passed in 0.16s ==============================

$ uv run ruff check tests/public_api/test_do_annotation_coverage.py
All checks passed!

Refs gh-333-annotation-coverage