Route browser fs and logs endpoints directly to the VM

[tnsardesai]

Summary

Adds fs and logs/stream to the default KERNEL_BROWSER_ROUTING_SUBRESOURCES prefixes, so every /browsers/{id}/fs/* operation (JSON, binary read/write, multipart upload, watch SSE) and /browsers/{id}/logs/stream is rewritten by the routing fetch to the browser VM's cached base_url with ?jwt= and no API-key Authorization.

Browser lifecycle/metadata, extensions, replays, telemetry/events, and anything else under logs/ stay on the control plane. KERNEL_BROWSER_ROUTING_SUBRESOURCES still overrides the list, and an empty value still disables routing.

Fixes one latent bug the new routing surfaced: createRoutingFetch builds a probe Request from the incoming body for cache sniffing, and that construction derives a multipart/form-data content-type carrying its own boundary. Copying that header onto the routed request while re-encoding the FormData body produced a boundary mismatch, and the VM rejected direct fs.upload calls with 400 failed to read form part. The routed init now drops an inherited content-type when the body derives its own (FormData, URLSearchParams, Blob) and the caller did not set one explicitly. fs.upload is the first routed multipart endpoint, so nothing previously routed was affected.

Query preservation, header stripping, signal propagation, stale-JWT eviction and the fallback body construction are otherwise unchanged; the added tests pin the fallback's existing body replay behavior for the body shapes the SDK produces.

Tests

tests/lib/browser-routing.test.ts:

• default prefix list and segment-boundary matching include fs and logs/stream, and still exclude telemetry/events, fsx/..., logs, logs/history, logstream, extensions, replays
• a routing-fetch level test proves logs/stream routes to the VM while logs, logs/history and logstream stay on the API origin
• fs JSON (listFiles), binary readFile, binary writeFile, multipart upload (indexed files[0][…] field names), fs/watch/{id}/events SSE and logs/stream SSE route to the VM with the query preserved, ?jwt= appended and no authorization header
• a routed multipart request parses with request.formData(), which fails on the boundary mismatch described above (verified: the test fails without the fix)
• logs.stream aborts stay wired to the routed request's signal
• stale-JWT fallback replays the writeFile binary body byte for byte and the upload multipart body on the control plane with bearer auth, and evicts the route
• env override keeps fs/logs on the control plane; empty env disables routing
• control-plane subresources (telemetry/events, replays, extensions) keep the API origin and bearer auth

Ran locally: jest (421 passed, 227 skipped), prettier --check ., eslint ., tsc, ./scripts/build.

Live validation

Ran against staging with real headless browsers: fs.writeFile, fs.readFile, fs.listFiles, fs.upload (two entries), fs.watch.start/events/stop and logs.stream all hit https://<browser-host>/browser/kernel/...?jwt=... and returned the expected data, with uploaded files reading back with the correct per-entry contents. telemetry/events and the browser delete stayed on the control plane. The fs.upload failure above was found this way and re-verified as fixed; re-validated after switching the default to logs/stream.

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Note

Medium Risk
Changes default request paths for fs and streaming logs (data and uploads) and touches request header/body handling for routed multipart; mitigated by control-plane fallback, env allowlist, and broad test coverage.

Overview
Default direct-to-VM routing now includes fs and logs/stream, so browser file operations (list/read/write/upload, watch SSE) and live log SSE go to the session VM with JWT query auth instead of the API control plane. Other paths under logs/, plus telemetry/events, extensions, and replays, stay on the API origin.

Multipart fix: When rebuilding the routed fetch body, inherited content-type from an intermediate Request could disagree with a re-encoded FormData boundary and break fs.upload. Routed requests now drop content-type when the body is FormData/URLSearchParams/Blob and the caller did not set it explicitly, so the runtime derives a matching boundary.

Tests cover prefix matching, VM routing for binary and multipart bodies, SSE abort wiring, stale-JWT fallback with body replay, and KERNEL_BROWSER_ROUTING_SUBRESOURCES overrides.

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