The record/replay harness stored a streamed provider response as one
buffered body, so a replayed stream arrived coalesced and the
/v1/messages streaming test could not be edge-wired. Keep each SSE
transfer chunk in the bundle in the order the provider sent it (a new
streamed response shape at BUNDLE_FORMAT_VERSION 4) so replay reproduces
the provider's split points, the recorded usage chunk keeps its
position, and a mid-stream upstream error replays as the same
mid-stream error rather than a clean body.
Resolves LIT-5742
Replaces the test-side fixture transport with an in-process provider-edge
HTTP server the proxy's deployments point their api_base at. Record forwards
provider calls verbatim and writes them to the bundle; replay answers them
from the bundle with zero provider calls while key auth, routing, cost
calculation, and spend-log writes still execute against the live proxy and
database. Drift comes back as HTTP 599 naming the computed and closest
recorded keys. Request headers are never stored and responses are kept
byte-identical between modes from the proxy's side of the socket.
E2E_FIXTURE_MODE selects the transport every e2e client is built on: live
(default, unchanged behavior), record (pass through to the live proxy while
writing every interaction to a fixture bundle), or replay (serve every
interaction from the bundle with no proxy and no provider spend). Both new
transports fulfil the existing Transport protocol, so no test changes shape.
A bundle is a directory with a manifest (record timestamp, harness version,
format version) and one JSON file per interaction, grouped per test in call
order. Replay against a manifest older than seven days hard-fails at
collection time naming the bundle age. Record always wipes and never reads
the previous bundle, refusing to wipe a directory that is not a bundle.
Auth header values are redacted on write; uploads store a sha256 digest.
unique_marker() becomes deterministic per test in record/replay modes so a
replay run regenerates exactly the requests the record run sent.
Content-based match keys, streaming chunk fidelity, and provider-scoping are
follow-ups (LIT-5741, LIT-5742, LIT-5745).
