fix: report duplicate plugin names as import errors

[jinoo7099]

Description

Report duplicate Airflow plugin names as plugin import errors.

When multiple plugins define the same name, Airflow already skips the later duplicate plugin, but the conflict was only logged as a warning. This made the issue hard to discover from the UI/API because only one plugin appeared in the plugin list.

This change keeps the existing behavior of skipping duplicate plugin names and continuing to load subsequent plugins, but also records the duplicate as a plugin import error so it is visible from the plugin import errors endpoint/UI.

• closes: User is able to register multiple plugins using same name #55140

Testing

sample plugin code

from airflow.plugins_manager import AirflowPlugin
from fastapi import FastAPI

my_app = FastAPI()

@my_app.get("/ping")
async def ping():
    return {"message": "pong"}

class MyFastAPIPlugin(AirflowPlugin):
    name = "my_fastapi_plugin"
    fastapi_apps = [
        {
            "name": "example_api",
            "app": my_app,
            "url_prefix": "/example"
        }
    ]

@my_app.get("/hello")
async def ping():
    return {"message": "Hello"}

class MyFastAPIPlugin2(AirflowPlugin):
    name = "my_fastapi_plugin"
    fastapi_apps = [
        {
            "name": "example_api",
            "app": my_app,
            "url_prefix": "/example"
        }
    ] 

• Before : Import error is not shown.

• After : Duplicate Airflow plugin names are shown as an import error.

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Was generative AI tooling used to co-author this PR?

• Yes — OpenAI Codex (GPT-5)

Generated-by: OpenAI Codex (GPT-5) following the guidelines

[potiuk]

LGTM