feat(weave): send client-capability header on ingest (Python + Node)

[ro31337]

JIRA Issue(s)

https://coreweave.atlassian.net/browse/WB-35951

Description

Both the Python and Node SDKs now send an X-Weave-Client-Capabilities header on every request they make to the Weave trace server. The value is a small comma-separated list of the sampling-relevant capabilities this SDK build guarantees — today trace_id_on_end,eval_child_meta, matching the two recent changes that put trace_id on every call message and tag evaluation child calls with eval metadata.

The header is groundwork for a future server-side ingest sampler. To sample the calls model safely the server has to drop whole traces consistently and never shred evaluations, which is only true once a client sends trace_id everywhere and marks its eval calls. The server cannot infer that from the data alone — an unmarked call is ambiguous (a normal call, or an eval child from an older client) — so the client states it directly, and the sampler leaves traffic without the header unsampled. Nothing reads the header yet; this mirrors the X-Weave-Trace-Id header added recently for the same future sampler.

Why this approach

A capability list rather than an SDK version keeps the server out of mapping versions to behavior: the Python and Node SDKs version independently, and the relevant behavior landed across several releases. It is advisory, not a gate — a missing or unknown capability just means "don't sample," never a rejected request. It is set once in the shared request-header builder, so it rides every request (it describes the client, not one trace); a future sampler simply reads it on the ingest paths.

Testing

Python: new tests assert the header is present on single-call and on batch upsert requests; the full trace_server_bindings suite passes (64). Node: a new clientApi test asserts init() wires the header onto the trace-server client; the full suite passes (313). The VCR cassette matcher now ignores this header the same way it already ignores the volatile client_version body field, so the recorded live/ cassettes still match. Lint, format and typecheck (cjs + esm) are clean.

Backward compatibility

Additive and safe both ways: an older server ignores the unknown header, and an older client simply doesn't send it. The Stainless transport and the one-shot login health check are out of scope, matching the X-Weave-Trace-Id change. One follow-up to track: if the Stainless transport later becomes the default, this header and X-Weave-Trace-Id must be ported to it together, otherwise the sampler would see header-less traffic from the new default client.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[ro31337]

^ is it too wordy?

[ro31337]

Splitting the TS and Python changes into separate PRs per review feedback (keeps reverts simple): Python #7350, Node #7351. Both are drafts with green CI. Closing this combined PR in favor of those.