Migrate tensorzero implementation from native inference to OpenAI, and add quickstart

[aaronpolhamus]

This PR:

1. Cleans up examples/tensorzero, simplifying the MCP server setup (thanks for all the help on this @GMakarenko)
2. Migrates the previous implementation of tensorzero from the native inference endpoint to the OpenAI interface, significantly simplifying the code
3. Add fast-agent quickstart t0 / fast-agent quickstart tensorzero

In addition to the core rest runner, smoke tests pass, too:

================================================================================================================================================ test session starts ================================================================================================================================================
platform darwin -- Python 3.13.5, pytest-8.4.1, pluggy-1.6.0
rootdir: /Users/aaronpolhamus/code/vest2/fast-agent
configfile: pyproject.toml
plugins: anyio-4.9.0, asyncio-1.1.0, cov-6.2.1
asyncio: mode=Mode.STRICT, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collected 3 items                                                                                                                                                                                                                                                                                                   

tests/e2e/smoke/tensorzero/test_agent_interaction.py .                                                                                                                                                                                                                                                        [ 33%]
tests/e2e/smoke/tensorzero/test_image_demo.py .                                                                                                                                                                                                                                                               [ 66%]
tests/e2e/smoke/tensorzero/test_simple_agent_interaction.py .                                                                                                                                                                                                                                                 [100%]

================================================================================================================================================ 3 passed in 59.99s =================================================================================================================================================