refactor: move missing tests to test directory (#1892)

Move the test_context.py under the main tests directory, and fix the
code.

The problem was that the function captures the initial values of the
context variables and then restores those same initial values before
each iteration. This means that any modifications made to the context
variables during iteration are lost when the next iteration starts.

Error was:

```
====================================================== FAILURES =======================================================
______________________________________ test_preserve_contexts_across_event_loops ______________________________________

    @pytest.mark.asyncio
    async def test_preserve_contexts_across_event_loops():
        """
        Test that context variables are preserved across event loop boundaries with nested generators.
        This simulates the real-world scenario where:
        1. A new event loop is created for each streaming request
        2. The async generator runs inside that loop
        3. There are multiple levels of nested generators
        4. Context needs to be preserved across these boundaries
        """
        # Create context variables
        request_id = ContextVar("request_id", default=None)
        user_id = ContextVar("user_id", default=None)

        # Set initial values

        # Results container to verify values across thread boundaries
        results = []

        # Inner-most generator (level 2)
        async def inner_generator():
            # Should have the context from the outer scope
            yield (1, request_id.get(), user_id.get())

            # Modify one context variable
            user_id.set("user-modified")

            # Should reflect the modification
            yield (2, request_id.get(), user_id.get())

        # Middle generator (level 1)
        async def middle_generator():
            inner_gen = inner_generator()

            # Forward the first yield from inner
            item = await inner_gen.__anext__()
            yield item

            # Forward the second yield from inner
            item = await inner_gen.__anext__()
            yield item

            request_id.set("req-modified")

            # Add our own yield with both modified variables
            yield (3, request_id.get(), user_id.get())

        # Function to run in a separate thread with a new event loop
        def run_in_new_loop():
            # Create a new event loop for this thread
            loop = asyncio.new_event_loop()
            asyncio.set_event_loop(loop)

            try:
                # Outer generator (runs in the new loop)
                async def outer_generator():
                    request_id.set("req-12345")
                    user_id.set("user-6789")
                    # Wrap the middle generator
                    wrapped_gen = preserve_contexts_async_generator(middle_generator(), [request_id, user_id])

                    # Process all items from the middle generator
                    async for item in wrapped_gen:
                        # Store results for verification
                        results.append(item)

                # Run the outer generator in the new loop
                loop.run_until_complete(outer_generator())
            finally:
                loop.close()

        # Run the generator chain in a separate thread with a new event loop
        with ThreadPoolExecutor(max_workers=1) as executor:
            future = executor.submit(run_in_new_loop)
            future.result()  # Wait for completion

        # Verify the results
        assert len(results) == 3

        # First yield should have original values
        assert results[0] == (1, "req-12345", "user-6789")

        # Second yield should have modified user_id
        assert results[1] == (2, "req-12345", "user-modified")

        # Third yield should have both modified values
>       assert results[2] == (3, "req-modified", "user-modified")
E       AssertionError: assert (3, 'req-modified', 'user-6789') == (3, 'req-modified', 'user-modified')
E
E         At index 2 diff: 'user-6789' != 'user-modified'
E
E         Full diff:
E           (
E               3,
E               'req-modified',
E         -     'user-modified',
E         +     'user-6789',
E           )

tests/unit/distribution/test_context.py:155: AssertionError
-------------------------------------------------- Captured log call --------------------------------------------------
ERROR    asyncio:base_events.py:1758 Task was destroyed but it is pending!
task: <Task pending name='Task-7' coro=<<async_generator_athrow without __name__>()>>
================================================== warnings summary ===================================================
.venv/lib/python3.10/site-packages/pydantic/fields.py:1042
  /Users/leseb/Documents/AI/llama-stack/.venv/lib/python3.10/site-packages/pydantic/fields.py:1042: PydanticDeprecatedSince20: Using extra keyword arguments on `Field` is deprecated and will be removed. Use `json_schema_extra` instead. (Extra keys: 'contentEncoding'). Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.10/migration/
    warn(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================== short test summary info ===============================================
FAILED tests/unit/distribution/test_context.py::test_preserve_contexts_across_event_loops - AssertionError: assert (3, 'req-modified', 'user-6789') == (3, 'req-modified', 'user-modified')

  At index 2 diff: 'user-6789' != 'user-modified'

  Full diff:
    (
        3,
        'req-modified',
  -     'user-modified',
  +     'user-6789',
    )
```

[//]: # (## Documentation)

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/distribution/utils/context.py

@@ -29,6 +29,11 @@ def preserve_contexts_async_generator(
                     context_var.set(initial_context_values[context_var.name])
 
                 item = await gen.__anext__()
+
+                # Update our tracked values with any changes made during this iteration
+                for context_var in context_vars:
+                    initial_context_values[context_var.name] = context_var.get()
+
                 yield item
 
             except StopAsyncIteration: