feat(public-api): add /v1/sessions/:id/runs endpoint for AG-UI prompt delivery

[jeremyeder]

Jira: RHOAIENG-51908

Problem

The public API has no endpoint for sending prompts to a running session. The only way to deliver a prompt to a runner is:

1. Through the frontend's backend proxy at /api/projects/:name/agentic-sessions/:id/agui/run
2. Directly to the runner pod on port 8001 via port-forward or in-cluster access

Option 1 requires a browser. Option 2 bypasses the UI entirely — the frontend never sees the AG-UI event stream, so work happens invisibly.

Impact

• The MCP server (ambient-code/mcp) and CLI (acpctl) cannot send prompts to sessions through any API
• The acp-compile workflow (submit a plan for autonomous execution) has no proper delivery mechanism
• Direct pod access creates invisible work that the UI can't display

Proposed Solution

Add POST /v1/sessions/:id/runs to the public API that proxies to the runner's AG-UI endpoint, similar to how the backend already proxies at /api/projects/:name/agentic-sessions/:id/agui/run.

This enables:

• MCP server to send prompts via API
• CLI to submit work via acpctl
• Any API client to interact with sessions

The endpoint should:

• Accept the same AG-UI RunAgentInput JSON body the runner expects
• Stream SSE events back to the caller
• Require the same auth (Bearer token + X-Ambient-Project header)

Context

Discovered while trying to use the acp-compile skill to submit a plan to a running session on a kind cluster. The only way to deliver the prompt was curl directly to the pod, which cut the UI out completely.

[github-actions]

Amber Change Summary

The following files were modified:

 components/public-api/handlers/integration_test.go | 151 +++++++++++++++++++++
 components/public-api/handlers/middleware.go       |   7 +-
 components/public-api/handlers/sessions.go         |  74 ++++++++++
 components/public-api/main.go                      |   7 +-
 components/public-api/types/dto.go                 |  11 ++
 5 files changed, 245 insertions(+), 5 deletions(-)

Next Steps:

• Review that changes match the issue description
• Verify no scope creep or unintended modifications
• A PR will be created shortly for formal review

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[github-actions]

🤖 Amber has created a pull request to address this issue: #755

The changes are ready for review. All automated checks will run on the PR.

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[github-actions]

Inactive for 30 days. Will close in 7 days unless there's activity.