fix(pi): pass model samplingParams into the isolated models.json

[drewstone]

Defect

src/backends/pi-inference-transport.ts copies only SAFE_MODEL_FIELDS from the operator's ~/.pi/agent/models.json model entry into the isolated models.json each pi run reads.
That allowlist dropped samplingParams.
Pi merges a model's samplingParams object into every request body (Object.assign(params, options.samplingParams) in pi's openai-completions transport; Model.samplingParams?: Record<string, unknown> in pi's types).
It is the only way to send body-level gateway options such as {"gateway": {"streamIdleTimeout": 180000}} to router.tangle.tools, because the router reads gateway options from the body only.

Without it, a Claude model that pauses longer than the router's 30 s stream-idle default has its stream closed by the router ("sent nothing for 30s and the router closed the stream").
Pi retries, and the bridge's accounting check (src/backends/pi.ts accountingMatched requires failedRequests === 0) then refuses the whole session.

Measurement

2026-09-02, fleet host running cli-bridge 0.3.0 (origin/main 0299595): 23 of 25 single-inference workers refused in one run, 2 stream-idle closures per refused session.

Change

• Add samplingParams to SAFE_MODEL_FIELDS. Body-level request parameters only; auth never rides here.
• Factor the two identical allowlist copy loops (explicit models.json path and pi catalog path) into copySafeModelFields, which rejects a samplingParams value that is not a plain object with the file's existing BackendError('not_configured') style. Pi spreads the value into the request body, so an array or scalar would produce a malformed upstream request.

Test

tests/pi-inference-isolation.test.ts:

• carries model samplingParams into the isolated models.json: a model entry with samplingParams: { gateway: { streamIdleTimeout: 180000 } } survives into resolved.modelConfig and into the models.json written by provisionPiInferenceTransport.
• rejects model samplingParams that are not a plain object: an array value is refused before pi auth is invoked.

Local results (macOS): the two new tests pass; pnpm build (tsc --noEmit) exits 0.
The same file has 2 pre-existing failures on pristine origin/main on macOS (/proc/self/environ missing; /tmp realpath prefix), unrelated to this change.

Note

The fleet host applied the same one-line allowlist change locally on 2026-09-02, pending this PR.