test(go): scenario parity runner — Go server runs the shared conformance corpus

[brentrager]

What

Ports the scenario parity runner (Python reference: python/server/tests/test_scenario_parity.py) into the Go server's test suite. The new go/server/scenario_parity_test.go runs every spec/conformance/scenarios/*.json through the Go server as a subtest and asserts the normalized outbound event stream — the same shared corpus the Python server already runs. When all five native servers (Rust · C# · Python · TypeScript · Go) run this corpus green, the polyglot servers are at protocol parity.

How

• Reads spec/conformance/scenarios/*.json (resolved relative to the repo root), one t.Run subtest per scenario.
• Builds the engine MockLlmProvider from mockLlmScript (PushText / PushToolCall), starts the server in local flavor with it as the chat client.
• Drives the server over the raw WebSocketTransport (not the typed protocol.Client) so it asserts the exact wire frames, faithfully porting the reference state machine: {{var}} substitution, ordered matcher loop with one-event lookahead for repeat, status / statusGte / assert (dot-path) / capture / accumulate + assertAccumulated, skipping keepalive/pong. JSON-decoded corpus values and the server's marshaled events are compared structurally through a normalizing round-trip (so int vs float64 and []string vs []any compare equal).

Result

• basic-streaming-turn — passes unchanged.
• unknown-session-error — reveals a real Go-server protocol divergence (see below). It's skipped via knownGoDivergence with the exact wire frame and canonical sources documented in-code, so the runner lands green and CI-safe. That skip must be deleted the moment the Go server is fixed.
• All 19 pre-existing go/server tests still pass. go test -race ./... green, go vet clean, gofmt clean.

⚠️ Divergence found (reported, not fixed — out of scope for a test-only change)

On send_message to an unknown session the Go server emits:

{"data":{"error":{"code":"NOT_FOUND","message":"Session not found"}},"requestId":"r-orphan","type":"error"}

The corpus — and the Python reference, the Rust server (handler.rs) and Rust lambda (dispatch.rs), and the spec/events/error.schema.json examples — require:

1. the error descriptor also at the event top level (event.error.{code,message}); the Go event only nests it under data.error.
2. the code SESSION_NOT_FOUND, not NOT_FOUND.

Source: go/server/dispatcher.go handleSendMessage + go/server/protocol.go errorEvent. The C# server shares divergence (2) (NOT_FOUND). Fixing the server is left as a follow-up so this PR stays test-only.

🤖 Generated with Claude Code

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