fix: preserve comments in minimal read filter

[Daishuyuan]

Summary

This change keeps comments visible when rtk read --level minimal is used.

• make MinimalFilter only normalize trailing whitespace and repeated blank lines
• move comment stripping into a dedicated helper used by AggressiveFilter
• add regression tests for both minimal and aggressive behavior

Why

minimal was still dropping source comments, which made rtk read less useful when the reader needed inline notes, rationale, or TODO context from the file.

Verification

• cargo +stable test test_minimal_filter_preserves_comments -- --nocapture
• cargo +stable test test_aggressive_filter_still_removes_comments -- --nocapture
• cargo +stable test test_json_no_comment_stripping -- --nocapture