[triage:tooling-01-delayed-chunks-identical-no-delay-no-chunking] `/delayed-chunks` and `/chunked` are byte-identical, buffered, and never chunked or delayed

Imported from Capsem triage report `tooling-01-delayed-chunks-identical-no-delay-no-chunking.md`.

- Severity: `medium`
- Category: `bug (mock divergence)`
- Area: `capsem-mock-server`
- Location: `crates/capsem-mock-server/src/main.rs:414-418`
- Confidence: `verified`

## Summary
The mock advertises two distinct streaming-shaped endpoints, `/chunked` and `/delayed-chunks`, but they share a single route arm that returns one fully-buffered body with a fixed `Content-Length` and no inter-chunk delay. Tests that depend on either chunked transfer-encoding *or* a time gap between chunks (the two behaviors the endpoint names promise) will pass against a mock that exhibits neither, masking real proxy bugs in chunk reassembly and timing.

## Evidence
```rust
(&Method::GET, "/chunked") | (&Method::GET, "/delayed-chunks") => response(
StatusCode::OK,
Bytes::from_static(b"chunk-0\nchunk-1\nchunk-2\nchunk-3\n"),
"text/plain; charset=utf-8",
),
```
`response()` (line 639) calls `response_builder()` (line 678), which unconditionally sets `CONTENT_TYPE` and `CONTENT_LENGTH` headers, and wraps the body in `Full::new(body)` (line 689). A `Full` body with an explicit `Content-Length` is delivered as a single non-chunked HTTP/1.1 response. No `tokio::time::sleep` is present on this path, unlike `/mcp` (line 547) which does delay via `mcp_payload_should_delay`. So:
- `/delayed-chunks` introduces zero delay — identical wall behavior to `/chunked`.
- Neither uses `Transfer-Encoding: chunked`; both send `Content-Length: 32`.

## Impact
The MITM proxy's chunk-boundary and streaming-timeout handling is exercised by these endpoints. A buffered, length-delimited single body cannot reproduce: partial-chunk buffering, chunk framing parse, or read-timeout-between-chunks logic. A regression in the proxy's chunked-stream path would not be caught by tests hitting these endpoints, because the mock never actually chunks or delays.

## Suggested fix
Make `/chunked` emit a real `Transfer-Encoding: chunked` body (e.g. a `StreamBody`/`channel` body with multiple frames and no `Content-Length`), and make `/delayed-chunks` insert a `tokio::time::sleep` between frames. At minimum, split the two arms so `/delayed-chunks` delays. If chunked streaming is genuinely out of scope, rename/remove the endpoints so the advertised contract matches behavior.

## Triage
Confirmed from the local reviewed report in `/Users/elie/git/capsem/tmp/bugs/tooling-01-delayed-chunks-identical-no-delay-no-chunking.md`. Track implementation in the triage sprint; add regression coverage before fixing.

Start in crates/capsem-mock-server/src/main.rs at the route arms around lines 414-418, then read response() and response_builder() around lines 639-689 and compare the /mcp delay path near line 547. Add regression coverage before changing the mock. Done means the advertised chunking and delayed-chunk behaviors are observable, or the endpoints are renamed or removed to match their actual contract.

Written by the indexing model from the issue text.