Motivation & Context

Scheduling stream initialization with asyncio.create_task() currently breaks ResponseStream, even though its input accepts an awaitable. Task and Future sources are treated as iterators instead of being awaited, so callers get an AttributeError during iteration or RuntimeError: Inner stream not available during finalization.

Description & Review Guide

• What are the major changes? Use the existing isawaitable() check when resolving a stream source. Extend the constructor and from_awaitable() tests to cover coroutine, Task, and Future inputs.
• What is the impact of these changes? Pre-scheduled stream initialization now works through the existing API. Async iterable sources still take precedence, and coroutine handling is unchanged. The four new Task/Future cases failed before the fix; all 312 tests in test_types.py pass with it. Package build, package lint, and the repository pre-commit hooks pass, as do all five configured type checkers on the changed test file and strict Pyright on the changed source file. Full-package Pyright is not green in the minimal local environment: optional provider packages are absent. The full provider test suites were not run.
• What do you want reviewers to focus on? Stream-source resolution and preserving the behavior of values that are both awaitable and async iterable.

Related Issue

Fixes #8330

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.