[feat][evaluation] support a2a_agent and custom_agent eval target types on the OpenAPI experiment-create path

[jamesonics]

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feat

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[feat][evaluation] OpenAPI 创建实验链路支持 a2a_agent 与 custom_agent 评测对象类型

(Optional) More detailed description for this PR

en:
The web link already supports two eval target types — A2A Agent (a2a_agent, type=9) and the in-house
long-connection agent (custom_agent, type=10) — and their execution operators are registered downstream. The
OpenAPI experiment-create path, however, rejected them: mapOpenAPIEvalTargetType only handled 6 types, so
a2a_agent / custom_agent fell through to default and were blocked both by the up-front
SubmitExperimentOApi validation and by the convertor.

This PR wires the OpenAPI path to the same service layer the web link uses:

• mapOpenAPIEvalTargetType: add cases a2a_agent -> A2AAgent, custom_agent -> CustomAgent.
• supportedOpenAPIEvalTargetTypes: add both so the up-front validation and the error message (the
  supported-types list) stay in sync.
• Unit tests for the two new mappings, plus an end-to-end convertor test covering custom_agent (cluster +
  agent_connection passthrough) and a2a_agent (source_target_id reference).

No IDL change (the OpenAPI string enums and EvalTargetContent sub-fields already exist) and no operator
change (already registered downstream). faas_http (an AccessProtocol, not an EvalTargetType) is still
correctly rejected.

Verified on a PPE swimlane: a2a_agent / custom_agent experiments are created successfully (with field
mappings), and faas_http is rejected with the supported-types list now containing all 8 types.

zh(optional):
网页端已支持 A2A Agent（a2a_agent，type=9）和内场长链接
Agent（custom_agent，type=10），执行算子也已在下游注册，但 OpenAPI
创建实验链路一直拒绝这两类——mapOpenAPIEvalTargetType 只处理 6 类，a2a/custom 落入 default 被拦。本 PR
在转换层补两个 case + 同步白名单，让 OpenAPI 链路接到与网页端相同的服务层。不改 IDL、不改算子；faas_http
仍正确被拒。已在 PPE 泳道验证：a2a/custom 实验创建成功，faas_http 报错且枚举列表已含 8 类。

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@           Coverage Diff           @@
##             main     #560   +/-   ##
=======================================
  Coverage   77.81%   77.82%           
=======================================
  Files         670      670           
  Lines       76100    76119   +19     
=======================================
+ Hits        59217    59238   +21     
+ Misses      13421    13420    -1     
+ Partials     3462     3461    -1     

Flag	Coverage Δ
unittests	77.82% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...uation/application/convertor/experiment/openapi.go	83.64% <100.00%> (+0.03%)	⬆️

... and 11 files with indirect coverage changes

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