Promote google.adk.cli.fast_api.get_fast_api_app to public namespace

[olliefr]

Link to Issue or Description of Change

Link to an existing issue:

• Closes: TBA
• Related: #4500

Problem:
As described in the aforementioned issue, a method from one of the ADK modules is being used in multiple places in the official ADK documentation yet there is no API Reference docs generated for it at the moment.

Solution:
I'm simply following the plan devised by @klateefa as detailed in the aforementioned issue.

This is my first PR to the ADK repo so help is welcome.

My understanding is that once this PR is merged, the API Reference docs can be regenerated and then they will pick up and include the docstrings for the method in question.

I'm also going to submit a related PR to the adk-docs repo to include the module from this PR in the documentation build.

Testing Plan

I run ./scripts/unittests.sh as directed by the instructions in CONTRIBUTING.md.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

A summary of passed pytest results:

============================== 4385 passed, 1 skipped, 2076 warnings in 119.03s (0:01:59) ==============================

--------------------------------------------------
Unit tests passed for all specified versions!
--------------------------------------------------
Cleaning up .unittest_venv...
Reactivating pre-existing venv: /home/oliver/github.com/olliefr/adk-python/.venv
(.venv) oliver@neon:~/github.com/olliefr/adk-python$

Manual End-to-End (E2E) Tests:

Once this PR is merged, followed by the PR in adk-docs, Python API Reference should include the docs for google.adk.cli.fast_api.get_fast_api_app method:

https://google.github.io/adk-docs/api-reference/python/google-adk.html#module-google.adk.cli

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

I will update this PR once I had submitted the one for adk-docs.

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