feat: request and execution traces across HTTP, WebSocket, and executor stages (#279 phase 2)

Problem statement / motivation

Parent: #279 (phase 2). Depends on phase 1 foundation #338.

Make one request followable end-to-end: extract incoming W3C context, hang every executor stage under a single execution span, inject the active context into upstream inference requests, keep streaming spans alive until body completion or disconnect, and record typed execution and delivery outcomes so that HTTP 200 never implies success.

Proposed solution

WIP

Acceptance criteria

• Valid inbound traceparent → http.server.request is its child and the mock upstream receives a traceparent whose parent is the gateway's http.client.request; invalid/absent → new root, no error.
• A two-round streaming cassette shows rehydrate, two inference_round (each with an http.client.request child), tool.execute (bounded agentic.tool.type), compaction when triggered, and persist under one agentic.execute.
• Proxy and executor routes, HTTP and WebSocket transports, Responses and Messages APIs produce the same span shape.
• Streaming spans close after the last chunk on success, upstream failure (error.type bounded), shutdown cancel, and mid-body client drop (delivery=disconnected, execution=cancelled).
• Three response.create messages on one socket → three distinct execution spans linked to one session span; a pipelined message records non-zero queue wait.
• SSE error under HTTP 200 → execution.outcome=failed, otel.status_code=ERROR.
• Attribute scanner: no input/output text, tool arguments, Authorization/x-api-key, raw URLs with query, or upstream error bodies in any exported attribute.
• Existing cassette, relay-ordering, and WebSocket tests pass unchanged; no new unbounded channel.

Alternatives considered

Non-goals

Metrics and histogram definitions (phase 3); sampling changes; OTLP logs; refactoring run_stream's channel (#244 owns it); capturing payloads even behind a flag.

Additional context

No response

Read phase 1 foundation issue #338 and trace the existing run_stream path; first run the cassette, relay-ordering, and WebSocket tests mentioned here. Map the proxy and executor routes plus HTTP and WebSocket transports before changing anything. Done means the listed span shapes, typed outcomes, streaming-close cases, attribute restrictions, and unchanged existing tests all pass.