fix: dev ui url works in sub-apps

[racinmat]

Fixes google/adk-web#218.
Blocked by #2556, it depends on it because of the base_url.
Supersedes #2254 because that did not get requested for review and adk-bot was not triggered there for some reason.
I didn't figure out any better way to do this. Basically we need to have a way how to provide a custom URL to the assets/config/runtime-config.json. I have tested with my project that it works well.
The redirect to dev-ui needed some fixes too, so I added fixes and tests for that.

testing plan

I have added test that demonstrates the functionality in a sub-app.

I ran the relevant tests to my code and here are the results, everything passes.

 pytest ./tests/unittests/cli/test_fast_api.py   
======================================================================================================================= test session starts =======================================================================================================================
platform win32 -- Python 3.12.8, pytest-8.4.1, pluggy-1.6.0
rootdir: C:\Projects\others\adk-python
configfile: pyproject.toml
plugins: anyio-4.9.0, langsmith-0.3.45, asyncio-1.1.0, mock-3.14.1, xdist-3.8.0
asyncio: mode=Mode.AUTO, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collected 36 items                                                                                                                                                                                                                                                 

tests\unittests\cli\test_fast_api.py ....................................                                                                                                                                                                                    [100%]

[racinmat]

@wyf7107 see how this PR is fixing the issues in the code, google/adk-web#218
and here the tests pass, you can verify it by running such test without the fix from my PR. It will fail.

[danowar2]

That's the place

[racinmat]

@wyf7107 Hi, would you be so kind and review this PR?

[racinmat]

@wyf7107 could you take a look?

[wyf7107]

Did you try to get a custom url to [assets/config/runtime-config.json](https://github.com/google/adk-web/blob/03b23e615db45ec912bf75aae1259cafad03c5d6/src/assets/config/runtime-config.json)? Want to understand the reasoning behind, if you are trying to use a proxy, then can you try adk web ----url_prefix, we recently added this.

In theory we don't expect users to override the value in that file since adk web and the api server wil talk to each other in the same server.

[racinmat]

How can you programatically and reasonably get the value to the config you mentioned? It's part of a library, libraries are installed by package manager without reasonable way of modifying their contents. Yes, locally I tried to modify the contents when I was debugging it, but that's unusable for regular usage.