fix(gateway): merge instead of overwrite agent.request_overrides

[gitabtion]

Problem

After init_agent() merges custom_providers extra_body into
agent.request_overrides, the gateway runner's per-turn setup on
line 15856 unconditionally overwrites it:

agent.request_overrides = turn_route.get(request_overrides) or {}

turn_route.get(request_overrides) is {} when no
service_tier is set (see _make_turn_route at line 3487), so
any custom_providers extra_body that was injected during
agent init is silently discarded.

Fix

Merge instead of overwrite: preserve whatever the agent already
has in request_overrides, and only layer on turn_route
overrides when they are non-empty.

Tested

• Confirmed custom_providers extra_body (e.g.
  content_filter: false for 360 provider) survives past the gateway
  per-turn setup when service_tier is not set.

[teknium1]

Thanks for isolating a real gateway reset bug. Current main merges matching custom-provider extra_body during initialization (agent/agent_init.py:1696), then discards it in the per-turn refresh (gateway/run.py:18394); the proposed copy-and-update directly addresses that sequence.

Problems

• There is no regression test for the gateway sequence being changed. tests/agent/test_custom_provider_extra_body.py:6 tests the init helper, while tests/gateway/test_fast_command.py:107 tests route construction, but neither reaches the post-init assignment at gateway/run.py:18394.
• This is only the reset sub-case. Current runtime and turn-route field whitelists still omit request_overrides at gateway/run.py:1895-1904 and gateway/run.py:3901-3910. The linked #53765 correctly identifies those as separate paths.

Suggested changes

• Add a focused gateway regression that verifies custom-provider extra_body survives the per-turn refresh, including coexistence with a service-tier override.
• Keep the PR explicitly narrow, or consolidate with the broader plumbing work; the one-line change should not be treated as covering model-switch or other runtime-resolution paths.

Automated hermes-sweeper review.