fix(edit_file): range mode replaces whole lines (#543)

[atlas-from-plumb]

Why

edit_file range mode (edits: [{start_line, end_line, new_string}]) glued a non-empty new_string that had no trailing newline onto the line after the range. An empty new_string deleted its lines cleanly, so the two cases behaved differently. Replacing one struct field with two this way merged the second new field with the next one and broke the build, and nothing in the response flagged it (#543).

Change

• internal/tools/range_edit.go: range mode now works in whole lines. A non-empty new_string without a trailing newline gets the line ending of the text it replaces, so a CRLF line stays \r\n. The EOF exception follows from the same rule. A range that runs to the end of a file with no final newline replaced unterminated text, so the replacement stays unterminated and the file still has no final newline. Append mode (start_line: -1) uses the same rule: a file that ends with a newline keeps one, and a file without one stays that way. The separator added before appending to such a file now uses the file's line ending (\r\n in a CRLF file) rather than a bare \n. Deletion (new_string: "") is unchanged. The atomic path and the apply_partial path both call applyRangeEdit, so both are covered. Anchor mode is not touched.
• internal/tools/edit_file.go: the new_string schema text now says range mode replaces whole lines and adds a missing trailing newline except at EOF of a file that has none. The start_line text is shorter so the pinned tools/list payload stays within its budget: 44,960 of 45,000 bytes, 2 more than on main.
• docs/tools.md: adds a short "Line-range edits" paragraph. CHANGELOG.md: one entry under 0.20.4 → Fixed.

Tests

New file internal/tools/range_edit_newline_test.go:

• TestApplyRangeEdit_WholeLines: 1→1, 1→2, 2→1 and N→0 lines, each with and without a trailing newline; first line; last line of a file with and without a final newline, including end_line -1 and a capped end_line; CRLF; empty file.
• TestApplyRangeEdit_AppendWholeLines: append to LF and CRLF files, with and without a final newline.
• TestEditFile_RangeEdit_IssueRepro: the struct-field case from the issue, run end to end through Execute.
• TestEditFile_RangeEdit_BatchAndCRLF: four sequential range edits (replace, grow, delete, append) in one call on an LF file and a CRLF file, run through both the atomic and the apply_partial paths.

With range_edit.go reverted to origin/main, 22 of the new subtests fail. The cases that pass there are the positive controls: explicit trailing newlines, deletions, and preserving a missing final newline. Ten hand-applied mutants were all killed: no terminator; no CRLF detection; always \n; no empty-string guard; LF-only separator; no empty-file branch; verbatim append; whole-file ending instead of the replaced range's ending; checking for a \r\n suffix instead of \n; always-CRLF separator.

go test ./... -count=1 and go test -tags=integration ./internal/cli/ -count=1 both exit 0. golangci-lint is clean.

Closes #543

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