Tweak error message for insert_line

[enyst]

Tweak error message for the wrong insert_line parameter for str_replace_editor insert command, to try to avoid suggesting to the LLM that it's exactly a line.

Issue stems from confusing LLMs like this:

Let's re-read the error: "It should be within the range of lines of the file: [0, 110]". This sounds like line numbers are 0-indexed for insert_line in the API, but 1-indexed in view_range and cat -n.
If cat -n shows 111 lines, then 0-indexed lines are 0 to 110.
If I want to insert after the current last line (line 110 in 0-indexed, which is line 111 in 1-indexed cat -n output), I should use insert_line = 110.

The change proposed here and on the tool description are in testing.

Reference slack discussion

[xingyaoww]

LGTM - maybe @ryanhoangt could take a look too