fix(sidecar): strip ec_transfer_params from decode request in EPD mode

[revit13]

What type of PR is this?
/kind bug

What this PR does / why we need it:

In EPD (Encoder-Prefill-Decode) mode, every decode request arrived at the
decode pod carrying ec_transfer_params, causing vLLM to log a warning per
request:

(EngineCore) WARNING Got ec_transfer_params, but no ECConnector found.
Disabling ECTransfer for this request.

Requests succeeded (200 OK), but the warning is incorrect — the decode pod
has no EC connector and does not use these params.

Root cause

handleNIXLV2 receives a request body that runPDPipeline already
serialized with ec_transfer_params present (injected by the encoder
stage). The same completionRequest map is mutated in place for both the
prefill and decode stages. Encoder-only fields were never removed before
marshaling the decode body.

Fix

Delete ec_transfer_params from completionRequest when preparing the
decode stage, before setting kv_transfer_params and marshaling the decode
body (connector_nixlv2.go).

Test

Added a Ginkgo integration test in connector_nixlv2_test.go that drives
the full EPD path (encoder backend + prefill mock + decode mock) and
asserts:

• prefill request carries ec_transfer_params (correct — prefiller needs it)
• decode request does not carry ec_transfer_params (the fix)

The test fails before the fix and passes after.

Which issue(s) this PR fixes:
Fixes #1703

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

NONE