fix(deepagents): stop double line numbers in read_file for sandbox backends

[Alexei Vedernikov (alvedder)]

Sandbox read() used awk to emit cat-n–style NR + tab + line, and the read_file tool then ran formatContentWithLineNumbers again. Text reads showed two numeric columns e.g.

     1	     1	{
     2	     2		"name": "monorepo",
     3	     3		"version": "1.0.0",
...

This aligns sandbox text read() with FilesystemBackend: raw sliced lines from the backend; one numbering pass in the tool.

Changes

• buildReadCommand (libs/deepagents/src/backends/sandbox.ts): keep the same existence / empty-file checks and NR range (1-based inclusive slice from 0-based offset/limit); replace numbered printf with { print } so stdout is unnumbered lines only.
• Docs: Clarify on BaseSandbox.read, FilesystemBackend.read, CompositeBackend.read, and buildReadCommand that text read() returns a raw line slice; cat-n formatting is applied in read_file.
• Tests (libs/deepagents/src/backends/sandbox.test.ts):
  • Helper + test that read_file output has exactly one tab-separated “line number + body” segment per line (no extra leading number column) when using MockSandbox + createFilesystemMiddleware.
  • MockSandbox: simulate the real awk slice (NR >= start && NR <= end) and return raw joined lines (no prefixes).
  • Assert text reads use awk with { print }.
• @langchain/sandbox-standard-tests: rename the misleading read() case from “with line numbers” to reflect raw backend output.

Testing

• pnpm exec vitest run src/backends/sandbox.test.ts --typecheck (and related fs unit tests as appropriate).

Notes

• Binary reads are unchanged (still via downloadFiles).
• Callers that depended on numbered strings from backend.read() on sandboxes should switch expectations to raw lines (same as filesystem/state backends); numbering belongs to tool/UI layers that call formatContentWithLineNumbers.

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