fix: set Content-Length on passthrough response body (empty non-streaming responses)

[noyitz]

Summary

Fixes #307.

Non-streaming responses were delivered downstream with an empty body (Content-Length: 0) whenever the response body was not mutated by a plugin (passthrough). The root cause is an asymmetry between the request and response handlers: the request handler always re-asserts Content-Length (including its non-mutated branch, with an explicit "always set Content-Length to inform Envoy of the body size that will follow" comment), but the response handler only did so when the body was mutated.

The buffered response body is re-emitted to Envoy as a streamed body replacement (AddStreamedResponseBody). In FULL_DUPLEX_STREAMED mode Envoy relies on the ext_proc to declare the size of the replaced body, so the passthrough branch — which omitted Content-Length — produced an empty body downstream.

Why streaming was unaffected: SSE responses have no upstream Content-Length and are streamed normally. Why mutated responses were unaffected: the bodyMutated branch already sets Content-Length from the re-serialized body.

Change

• HandleResponseBody: add the symmetric else branch that sets Content-Length to len(responseBodyBytes) on the non-mutated (passthrough) body.
• generatePassthroughResponseBodyResponse: set Content-Length before building the headers response (this helper is only reached from HandleResponseBody with the complete buffered body, so len(responseBodyBytes) is the full body).

Test plan

• Updated TestHandleResponseBody_NoPlugins and TestHandleResponseBody_PluginNoBodyMutation to assert the passthrough Content-Length header (covers both passthrough entrypoints).
• make test (race) green.
• golangci-lint (v2.8.0) — 0 issues.
• go vet / gofmt clean.

[yossiovadia]

Independently verified the root cause — the asymmetry between request.go:119-120 (always sets Content-Length) and response.go (only sets it when bodyMutated == true) is confirmed in the code.

Fix is minimal and correct:

• HandleResponseBody else branch mirrors the request handler exactly.
• generatePassthroughResponseBodyResponse covers both call sites (no-plugins early return and JSON/SSE parse-failure fallback).
• Tests assert Content-Length is present in the passthrough HeadersResponse (was previously empty). Sort helpers already handle SetHeaders ordering.

No issues found. LGTM.