feat(harness): support once-time overrides on the streaming /run_sse path

[yaozheng-fang]

Summary

The harness app's /run_sse (the ADK streaming endpoint, mounted via the ADK api server) previously always ran the base agent, so streaming callers couldn't use the per-call overrides that /harness/invoke supports. This brings override support to the streaming path:

• Add a HarnessRunAgentRequest(RunAgentRequest) model with an extra optional harness field (a HarnessOverrides).
• Register a custom /run_sse and move it to the front of the router so it shadows ADK's default route (which the default is kept and delegated to).
• In the handler:
  • with harness override → stream a spawned agent (spawn_harness_agent(base, override) — base cloned + override applied), self-creating the session if absent; tool/skill load failures are surfaced as an SSE error event;
  • without override → delegate to ADK's original /run_sse unchanged (base path stays bit-for-bit stock behavior).

This gives the streaming endpoint the same override capability as /harness/invoke, while leaving the base path untouched.

Test

TestClient (exercises the lifespan):

• base /run_sse (no override) → base agent (HELLO.)
• override /run_sse (harness.system_prompt="…PINEAPPLE") → spawned agent (PINEAPPLE)

Deployed end-to-end (harness built from this branch) and verified through the gateway:

• run_sse no-override → HELLO; run_sse + --system-prompt → PINEAPPLE
• existing /harness/invoke (base + override) unaffected (MANGO / BANANA)

ruff clean. The AgentKit CLI side (agentkit invoke harness --protocol run_sse) sends the overrides as the harness field.

🤖 Generated with Claude Code