Support STREAMED response body

[Mohammad-nassar10]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Stacked on #24.
Getting a bug: Without per-chunk acks, Envoy stops forwarding response bytes after the first chunk, which surfaces to clients as ClientPayloadError: Not enough data to satisfy transfer length header.

• pkg/handlers/server.go: ack every non-EoS response body chunk so Envoy keeps streaming.
• pkg/handlers/response.go: always respond to response headers (in STREAMED mode Envoy blocks until we ack); on JSON parse failure, fall back to parseSSEResponseBody so streaming SSE responses still populate the response plugin pipeline; generateEmptyResponseBodyResponse now returns a single empty BodyResponse since the bytes were already forwarded by the per-chunk acks.
• config/charts/payload-processor/templates/istio.yaml: switch response_body_mode to STREAMED so Istio actually installs the EnvoyFilter.

Which issue(s) this PR fixes:

Fixes #

Release note (write NONE if no user-facing change):

NONE

[Mohammad-nassar10]

/cc @davidbreitgand @ronenkat

[ronenkat]

What is the relation to #4
is #4 needed? should we merge it?

[Mohammad-nassar10]

This is replacing #4 (have part of the changes of #4 with more fixes).

[ronenkat]

@Mohammad-nassar10 do you have a test that currently fails? and now passes with these changes?

[Mohammad-nassar10]

@Mohammad-nassar10 do you have a test that currently fails? and now passes with these changes?

@ronenkat no test that fails on main and passes with this PR, The bug is noticed in real Envoy in STREAMED mode (when we run the benchmark).

[ronenkat]

/approve

[ronenkat]

/lgtm