A2A managed endpoint returns empty/404 while runtime endpoint works

## Summary
The APIM-managed A2A endpoint path (`/ks/a2a-managed`) appears non-functional in our current configuration, while the policy-backed runtime path (`/ks/a2a-weather`) works reliably.

## Environment
- Repo: `Azure/apiops-cli`
- APIM target service: `bvt-7i6mlsfka5aec-tgt-apim`
- API: `src-a2a-weather-agent` (type `a2a`)
- API version: `2025-09-01-preview`

## Current Config (deployed)
`src-a2a-weather-agent` on target shows:
- `properties.path = ks/a2a-managed`
- `a2aProperties.agentCardBackendUrl = https://bvt-7i6mlsfka5aec-tgt-apim.azure-api.net/ks/a2a-weather/.well-known/agent-card.json`
- `jsonRpcProperties.backendUrl = https://bvt-7i6mlsfka5aec-tgt-apim.azure-api.net`
- `jsonRpcProperties.path = /ks/a2a-weather`

Operation resources also exist on the managed API:
- `GET /.well-known/agent-card.json`
- `POST /`
with policy present for `get-agent-card`.

## Repro
1. Publish extracted artifacts with `bvt-target.overrides.yaml`.
2. Call managed URL:
- `GET https://bvt-7i6mlsfka5aec-tgt-apim.azure-api.net/ks/a2a-managed/.well-known/agent-card.json`
- `POST https://bvt-7i6mlsfka5aec-tgt-apim.azure-api.net/ks/a2a-managed` with valid JSON-RPC body.
3. Call runtime URL:
- `GET https://bvt-7i6mlsfka5aec-tgt-apim.azure-api.net/ks/a2a-weather/.well-known/agent.json`
- `POST https://bvt-7i6mlsfka5aec-tgt-apim.azure-api.net/ks/a2a-weather`

## Actual
- Managed endpoint:
- `GET /ks/a2a-managed/.well-known/agent-card.json` => `200` with empty body
- `POST /ks/a2a-managed` => `404 Resource not found`
- Runtime endpoint:
- Returns valid agent card and JSON-RPC responses.

## Expected
Managed endpoint should be fully usable for discovery + invocation:
- Non-empty agent-card JSON on managed card route
- JSON-RPC request should route and return valid task response on managed root

## Impact
- APIM UI suggests using managed A2A URL, but runtime behavior requires using `/ks/a2a-weather`.
- Causes confusion for Foundry/tool onboarding and demo instructions.

## Notes / Potential Direction
A robust architecture likely requires `a2a-managed` pointing to an external runtime backend (App Service/Function/Container App) rather than APIM self-referencing paths, to avoid internal routing/preview gaps.