Fix Glancer to return correct FileWindow reflecting actual content read

[rbharvs]

Summary

• Fixes Glancer.glance() to return the actual FileWindow that was read from the file
• Previously returned the requested window instead of what was actually read
• For example, requesting 100 lines from an empty file now returns window with line_count=0 instead of line_count=100

Changes Made

• Added actual_window field to FileReadResult to track what was actually read
• Updated StreamingFileReader.read_window() to return actual window information
• Modified Glancer.glance() to use actual_window instead of requested window
• Updated FileWindow validation to allow line_count=0 for reporting empty reads
• Added comprehensive test suite (test_glancer_window_behavior.py) with 5 test cases covering edge cases

Test Plan

• All existing tests continue to pass (150/150)
• New tests verify correct behavior for empty files, partial reads, and offset beyond file end
• Manual verification with the original issue example works correctly
• All linting and type checking passes

Breaking Changes

• FileReadResult now requires an actual_window field
• FileWindow.line_count validation changed from ge=1 to ge=0

These are internal API changes that shouldn't affect most users of the public API.