chore: add type hints to FastAPI SSE generator functions

[Elbehery]

What does this PR do?

This change adds complete type annotations to sse_generator and
_preserve_context_for_sse functions in agents/fastapi_routes.py to satisfy
mypy's --disallow-untyped-defs requirement. These functions handle
Server-Sent Events streaming for the Agents API responses.

The sse_generator function converts async iterators to SSE format, while
_preserve_context_for_sse preserves request context across async tasks for
streaming responses. Type hints use AsyncIterator to properly type the
async generator functions.

A type ignore comment is used for the contextvars.Context.run pattern where
the type system has limitations in expressing the dynamic task creation. All
changes pass mypy strict checking and maintain full backward compatibility.

Test Plan

# 1. Run standard mypy check
  ./scripts/uv-run-with-index.sh run --group dev --group type_checking mypy \
    src/llama_stack_api/agents/fastapi_routes.py

  # 2. Run strict type checking (the key test)
  ./scripts/uv-run-with-index.sh run --group dev --group type_checking mypy --disallow-untyped-defs \
    src/llama_stack_api/agents/fastapi_routes.py

  # 3. Run all checks in one command
  ./scripts/uv-run-with-index.sh run --group dev --group type_checking mypy --disallow-untyped-defs \
    src/llama_stack_api/agents/fastapi_routes.py && \
  uv run ruff check \
    src/llama_stack_api/agents/fastapi_routes.py && \
  uv run black --check \
    src/llama_stack_api/agents/fastapi_routes.py && \
  ./scripts/uv-run-with-index.sh run --group dev --group type_checking mypy

[Elbehery]

cc @leseb @skamenan7 @derekhiggins

[cdoern]

is there a way for this testing (rather than by hand) to be done by mypy going forward?

[Elbehery]

is there a way for this testing (rather than by hand) to be done by mypy going forward?

I can add tests for this, historically I was just running the cmd :D

[Elbehery]

is there a way for this testing (rather than by hand) to be done by mypy going forward?

I added #5269

we can update the files section as we progress

[leseb]

remove the pyproject.toml entry of the file you're fixing

[leseb]

is there a way for this testing (rather than by hand) to be done by mypy going forward?

removing the ignore entry from pyproject is how we validate this with CI

EDIT: nevermind, was thinkig static type checking

was thinking static type checking

[mergify]

This pull request has merge conflicts that must be resolved before it can be merged. @Elbehery please rebase it. https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork