Treat edit_file filler args as absent for mode selection

[TheGreatAxios]

Problem

Models pad the unused edit_file mode's fields with fillers — start_line: 0, end_line: 0, old_string: "". parseEditFileMode treated any present line field as a line-range signal, rejected the call as "both edit modes", and models retried the identical call up to 8x. Production traces: 76% of gpt-5.6-terra edit_file calls rejected this way.

Fix

• Line fields that are null, non-integer, or < 1 count as absent for mode selection (0 is never a valid 1-based line); old_string: "" likewise counts as absent.
• Real mixed-mode calls (non-empty old_string AND line fields >= 1) are still rejected (preserves CL-4399), but the error now echoes the received values and names exactly which fields to drop so a retry can differ.
• The missing-mode and empty-old_string errors also state what was received.
• Tests cover filler-0 lines + old_string -> substring mode, empty old_string + valid range -> line-range mode, and the new error texts. Also fixed pre-existing lint errors in the touched files (type -> interface, non-null assertions) that CI would surface once their lint-cache entries invalidate.

Fixes CL-6900

[linear-code]

CL-6900