feat(sdk-generator): emit SSE contract tests in the contract-tests backend (TS + Python)

[calvin-archastro]

What changed

Follow-up to #43 (merged). #43 landed SSE method emission, typed events, the channel-harness SSE support, and the sample contract-test loop — but the contract-tests backend that the SDK repos regenerate from (--lang contract-tests-ts/py) still skipped streaming ops, so SDK regens shipped the generated stream() methods with no contract test. This wires SSE coverage into generateContractTests itself, for both backends.

• method-chain-builder: add buildStreamCalls — the streaming inverse of buildMethodCalls, sharing the resource walk so streaming ops get the same SDK accessor chains (client.v1.ai.chat.completions.stream).
• TypeScript → __tests__/contract/streams/<resource>.contract.test.ts: construct PlatformClient against the harness, registerStreamScenario with autoEmit (the harness synthesizes contract-valid event payloads, resolving $refs — no SDK-side fixtures needed), iterate stream(), assert the event order; plus a non-2xx → ApiError case.
• Python → tests/contract/streams/test_<resource>.py: same shape over AsyncPlatformClient.stream().
• Both gated by the existing opt-in (ARCHASTRO_RUN_CHANNEL_CONTRACT_TESTS): excluded from the default run, and the harness subprocess boots when the spec has channels or streams.

sequenceDiagram
    participant T as generated streams test
    participant H as harness control API
    participant C as PlatformClient.stream()
    participant S as harness SSE endpoint
    T->>H: registerStreamScenario(autoEmit each event)
    T->>C: for await ev of client.v1.ai.chat.completions.stream(input)
    C->>S: POST text/event-stream
    S-->>C: event frames (synthesized, contract-valid)
    C-->>T: {event, data} ... assert event order

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Scope

Generator-only (archastro-openapi). No runtime/SDK changes here.

Risk

Low. Additive: a new builder function + new emitted files behind the existing opt-in gate. The default contract-test run (REST, Prism-backed) is unchanged. Generator suite 299 (incl. 3 new tests covering TS + Python emission + the gating).

Testing

• Generator suite: 299 pass, tsc clean.
• Verified against the real public spec + archastro-js config: emits streams/ai.contract.test.ts (client.v1.ai.chat.completions.stream(...), autoEmit ×6, assert order, 402→ApiError) and tests/contract/streams/test_ai.py (async equivalent).

Runs green once deps land (unchanged from #43's follow-ups)

These tests are opt-in (excluded by default), so they don't affect CI. To execute:

• runtime streamSSE / stream_sse — archastro-js [codex] feat(sdk-generator): propagate OpenAPI docs into SDKs #34, archastro-python feat(python): emit response_type so generated calls deserialize into Pydantic models #28.
• registerStreamScenario on the harness client — TS: shipped via channel-harness (in feat(sdk-generator): emit SSE streaming methods for x-sdk-streaming #43); Python: needs adding to archastro-python's phx_channel/harness.py.