diagnostics: retain bounded structural summaries for successful and incomplete model streams

Summary

Mecatl retains content-free request manifests and sanitized evidence for failed or suppressed provider attempts. It deliberately retains no structural evidence for a successful HTTP/model stream.

That leaves a gap for reports such as #1639:

• the provider returns HTTP 200;
• the stream appears to finish;
• the normalized assistant text is nonsensical; and
• there is no durable evidence of whether Mecatl saw normal framing, a provider terminal event, an incomplete stream, or parser/decoder anomalies.

Add target-bound, durable, content-free structural stream evidence. Do not capture raw provider traffic.

Tracking: #1639

This issue improves future diagnosis of reports like #1639. It does not close, resolve, or claim to reproduce #1639.

Current behavior

network.attempt records failed or suppressed provider attempts. ADR 0255 explicitly excludes successful-attempt timing and protocol-phase evidence.

For a clean HTTP 200 stream that produces anomalous text, the existing failed-attempt path may contain no evidence at all.

Proposed behavior

For each provider attempt, retain a bounded structural summary that can answer questions such as:

HTTP status: 200
response framing: SSE
provider terminal frame observed: yes
protocol events accepted: 73
response bytes read: 14,220
local stream outcome: complete

The exact field names and closed vocabularies belong in the acceptance plan and decision record. The design must support at least these outcomes:

• complete;
• incomplete;
• stream error;
• cancelled; and
• evidence unavailable.

The debugger must describe these as observations, not causal conclusions.

For example:

Mecatl observed SSE framing and a provider terminal frame.

is permitted.

The gateway caused the bad response.

is not a conclusion this evidence can support on its own.

Required architecture constraints

• Provider adapters observe provider-specific stream structure.
• A provider-neutral, run-local observer carries only validated structural facts to the agent loop.
• The agent loop remains the sole event producer.
• The server relay remains the sole durable EventLog writer.
• Evidence is target-bound and available only through the session debugger.
• Ordinary gRPC, HTTP/SSE, Team, and event-log-readback streams do not expose it.
• Retried provider attempts remain separately correlated to the same logical turn.
• Missing support is explicit as unavailable; it is never presented as a healthy stream.
• The implementation must preserve streaming behavior: no ReadAll, no response buffering before delivery, and no change to TTFT, retry, cancellation, or stream-idle semantics.

Strict privacy boundary

This issue must never retain or expose:

• request, prompt, message, tool-schema, argument, or tool-result contents;
• response text or response fragments;
• raw errors or provider error codes;
• URLs, query strings, headers, cookies, tokens, credentials, or environment values;
• provider request/correlation IDs; or
• response-content hashes that become an offline content oracle.

This issue must not add --capture-wire, a capture directory, a raw-response debug mode, or a plaintext diagnostic artifact.

Why this is separate from request-manifest evidence

The request manifest answers:

What content-free request shape did Mecatl prepare?

This issue answers:

What content-free stream structure did Mecatl observe afterward?

A normal manifest cannot show whether the gateway ended an SSE stream correctly. A normal stream summary cannot show how many tool-schema bytes were advertised. The two records diagnose different parts of one turn.

Decision record

Architectural. ADR 0255 explicitly says successful attempts and protocol-phase timings are not measured. This issue requires a new or superseding decision record that defines the new retained-evidence boundary, its vocabulary, validation, visibility, and retention semantics.

Acceptance criteria

• A completed provider attempt produces one bounded, validated structural summary.
• A truncated or errored stream records incomplete/error structure without fabricating a normal terminal frame.
• Retry attempts remain distinguishable and correlate to their logical turn.
• Invalid provider-produced evidence is rejected and never persisted.
• All supported in-tree production provider families have offline fixture coverage.
• Durable relay persistence survives a disconnected client without making a diagnostic persistence failure fail inference.
• Ordinary client streams and public event-log readback omit this evidence.
• The target-bound debugger exposes bounded summaries and explicitly reports unavailable, incomplete, and truncated evidence.
• Debugger wording reports observations only and does not assign fault to Mecatl, a gateway, or a model.
• Engine API compatibility artifacts, changelog, documentation, and the superseding decision record are updated.