feat(sandbox): trace Python dataplane operations

[Mukil Loganathan (langchain-infra)]

Summary

• Trace Python sandbox dataplane operations as LangSmith tool runs for sync/async run, reconnect, write, read, and tunnel
• Wrap the public sandbox dataplane methods with a small @traceable-based helper and custom input/output processors for sanitized trace payloads
• Attach sandbox_id and sandbox_name metadata through the active tracing context while keeping command/file payload behavior unchanged: no sandbox id env injection, no env-key logging, and file contents stay out of trace inputs

JS changes are split into #2906.

Test Plan

• uv run ruff check python/langsmith/sandbox/_sandbox.py python/langsmith/sandbox/_async_sandbox.py python/tests/unit_tests/sandbox/test_sandbox.py python/tests/unit_tests/sandbox/test_async_sandbox.py
• uv run --extra pytest pytest tests/unit_tests/sandbox/test_sandbox.py tests/unit_tests/sandbox/test_async_sandbox.py

[github-actions]

JS perf benchmark

Lower is better. Noisy on shared runners — treat as a signal, not a gate.

Base64-heavy payload

Single large base64 string per message — the shape the worker-offload path is optimized for.
Payload: 2511.2 KB in / 5.2 KB out, 100 runs.

metric	main	this PR	delta
Wall time (ms)	2103.71	1913.43	-9.0%
createRun total (ms)	94.04	83.62	-11.1%
createRun p50 (ms)	0.64	0.62	-1.8%
createRun p95 (ms)	2.16	1.72	-20.3%
createRun p99 (ms)	33.05	27.43	-17.0%
createRun max (ms)	33.05	27.43	-17.0%
updateRun total (ms)	46.28	50.64	+9.4%
updateRun p95 (ms)	1.19	1.19	+0.3%
loop lag total (ms)	1100.38	779.19	-29.2%
loop lag p50 (ms)	0.09	0.10	+2.2%
loop lag p95 (ms)	5.12	3.83	-25.2%
loop lag p99 (ms)	21.57	12.05	-44.1%
loop lag max (ms)	107.09	125.00	+16.7%

Structural payload

Many small strings across a wide/nested object graph. Should bypass worker offload and use sync flush.
Payload: 1239.5 KB in / 13.3 KB out, 100 runs.

metric	main	this PR	delta
Wall time (ms)	1493.95	1498.22	+0.3%
createRun total (ms)	434.07	385.75	-11.1%
createRun p50 (ms)	3.43	3.30	-3.6%
createRun p95 (ms)	6.99	4.03	-42.3%
createRun p99 (ms)	13.74	21.62	+57.3%
createRun max (ms)	13.74	21.62	+57.3%
updateRun total (ms)	35.43	54.96	+55.1%
updateRun p95 (ms)	0.54	0.43	-19.7%
loop lag total (ms)	1054.66	1052.13	-0.2%
loop lag p50 (ms)	0.07	0.07	+0.4%
loop lag p95 (ms)	4.91	2.82	-42.6%
loop lag p99 (ms)	121.51	121.42	-0.1%
loop lag max (ms)	176.07	182.86	+3.9%