Infer agent-card.json from ADK agent

**Is your feature request related to a problem? Please describe.**
When I want to expose agents over a2a, using [`get_fast_api_app`](https://github.com/google/adk-python/blob/v1.8.0/src/google/adk/cli/fast_api.py#L1078-L1158), I [need to have `agent.json` stored in the directory](https://github.com/google/adk-python/blob/v1.8.0/src/google/adk/cli/fast_api.py#L1136-L1138) next to the python file with the agent card. The ADK agent defined in python already contains most of the information needed for the agent card, so it could be automatically provided without having to specify it explicitly as a json.

**Describe the solution you'd like**
I would like to have agent card inferred for the agent, maybe have some metadata per agent, stating if I want or don't want to expose it over a2a, and if needed, to provide information that are not already in ADK agent.

**Describe alternatives you've considered**
I considered automatically generating the json files, but this still incurs additional developer toil or requires additional CI setup to make it work.

**Additional context**
I don't think anything more is needed.

Start with get_fast_api_app in src/google/adk/cli/fast_api.py and the adjacent agent.json lookup described in the issue. Inspect which agent fields already provide agent-card information and what metadata would be needed to control A2A exposure or supply missing fields. Done means the design clearly defines how the card is inferred and served without requiring a separately stored JSON file.

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