fix(tool): do not count a trailing newline as a truncated line

[ANIRUDDHA ADAK (aniruddhaadak80)]

What

Truncate.output counted a phantom line for any output ending in a newline, so the "...N lines truncated..." figure it shows the model was one too high and the preview carried a blank entry.

Why it matters

That count is how the model judges how much of an output it is missing, printed right under "The tool call succeeded but the output was truncated." Almost every CLI and log file ends in a newline, so this was the common case, not an edge case.

The byte path is unaffected, and the full output written to disk is byte-identical — only the count and the trailing blank preview line change.

Verification

Fails before, passes after:

Expected to contain: "...90 lines truncated..."
Received:            "line0\n...\nline9\n\n...91 lines truncated..."

The new test is the existing truncates by line count case plus a trailing newline, so the two together pin both shapes:

(pass) truncates by line count [2.74ms]
(pass) counts a trailing newline as no line of its own [7.86ms]
 14 pass
 0 fail

All 13 pre-existing tests in the file still pass, including the byte-limit, tail-direction, and saved-output cases.

The change

     const direction = options.direction ?? "head"
-    const lines = text.split("\n")
+    // A trailing newline makes split yield a phantom final element, which
+    // would be kept as an empty preview line and reported as a removed one.
+    const lines = text.endsWith("\n") ? text.slice(0, -1).split("\n") : text.split("\n")

bun run typecheck clean; touched files are Prettier-clean (checked on LF-normalized copies — this Windows checkout has core.autocrlf=true, which makes Prettier flag every file in the repo).

Fixes #788

[vercel]

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