fix(output): propagate serialization and write failures

[Gautam Sharma (GautamSharma99)]

Summary

• make JSON, JSONL, and pretty JSON output helpers return serialization and I/O errors
• check short writes, file creation, writes, explicit closes, and status-output writes
• emit file-success status only after the data write and close complete successfully
• route every command output call through checked wrappers so failures terminate the CLI non-zero
• fix trace export's separate JSONL implementation to check marshal, write, short-write, and close failures

Closes #224.

Problem

The shared output helpers previously treated several failure modes as success:

• OutputJSON printed marshal/write errors but returned normally
• OutputJSONL ignored marshal, write, newline, and deferred-close errors
• PrintOutput ignored marshal and os.WriteFile errors entirely
• trace export had a second JSONL loop with unchecked marshal/write/close calls

In the JSONL path, the CLI printed {"status":"written"} even if an earlier write or the final close failed. Commands therefore exited with status 0 after producing a missing or truncated export.

Implementation

Error-returning output APIs

The following internal output functions now return error:

OutputJSON(data, path) error
OutputJSONL(items, path) error
PrintOutput(data, format, path) error

Errors include operation and path context while retaining the underlying error for errors.Is/errors.As.

JSON output now checks:

• json.MarshalIndent
• file writes
• stdout writes and short writes
• status writes

JSONL output now checks:

• file creation
• each item marshal
• each complete line write, including short writes
• explicit file close
• stdout and status writes

On a write failure, the file is closed best-effort and no success status is printed. On normal completion, close is performed and checked before success is reported.

Command propagation

All command call sites now use three checked command-layer wrappers. Those wrappers feed output failures through the CLI's existing ExitErrorf policy, producing an actionable stderr error and a non-zero process exit. This keeps existing command callbacks and successful output schemas unchanged while ensuring no helper error is ignored.

Most command-file changes in this PR are the mechanical replacement of:

output.OutputJSON(...)

with:

outputJSON(...)

The wrappers are the only command-layer callers of the error-returning output APIs.

Trace export

Trace export writes one JSONL file per trace without using the shared JSONL helper because it has per-trace extraction behavior. Its loop now:

• checks every marshal
• writes each JSON object and newline as one checked operation
• detects io.ErrShortWrite
• closes the file immediately on failure
• checks the successful final close before incrementing the exported count

The final count can no longer include a trace whose file failed to finish writing.

Behavior

Successful output and status payloads remain unchanged.

On failure, commands now behave like:

$ langsmith ... --output /missing/path/results.json
writing output: writing /missing/path/results.json: ...
$ echo $?
1

No "status": "written" line is emitted for failed writes or failed closes.

Tests

Added deterministic coverage for:

• unsupported values causing JSON marshal failure
• invalid/unwritable output paths
• pretty-output file failures
• partial writes returning io.ErrShortWrite
• explicit writer failures after file creation
• close failures
• closing a writer after a write failure
• suppression of success status after write or close failure
• subprocess-level confirmation that a command-layer output failure exits non-zero and names the failed path

Existing success tests now assert that the output helpers return nil.

Local verification:

• focused output failure tests
• command subprocess exit test
• go test ./...
• go test -race ./...
• go vet ./...
• make build
• git diff --check

make lint was not available locally because golangci-lint is not installed.