Preserve task types in asyncio.as_completed

[jw9829]

Fixes #15644. When as_completed() receives tasks, async iteration now preserves their Task[T] type instead of widening them to Future[T]. Regular iteration continues to yield coroutines.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

async-utils (https://github.com/mikeshardmind/async-utils)
+ src/async_utils/_merge_gens.py:211: error: Argument 2 to "anext" has incompatible type "Task[T] | None"; expected "T"  [arg-type]
+ src/async_utils/_merge_gens.py:262: error: Argument 2 to "anext" has incompatible type "Task[T] | None"; expected "T"  [arg-type]
+ src/async_utils/_merge_gens.py:314: error: Argument 2 to "anext" has incompatible type "Task[T] | None"; expected "T"  [arg-type]
- error: INTERNAL ERROR -- Please try using mypy master on GitHub:
- https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
- Please report a bug at https://github.com/python/mypy/issues
- version: 2.3.0
- note: use --pdb to drop into pdb
- Traceback (most recent call last):
-   File "/__main__.py", line 16, in console_entry
-     main()
-     ~~~~^^
-   File "mypy/main.py", line 154, in main
-   File "mypy/main.py", line 244, in run_build
-     res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
-   File "mypy/build.py", line 385, in build
-   File "mypy/build.py", line 1908, in create_metastore
-     mds: MetadataStore = SqliteMetadataStore(
-   File "mypy/metastore.py", line 200, in __init__
-   File "mypy/metastore.py", line 174, in connect_db
-     db.execute("PRAGMA journal_mode=WAL")
- sqlite3.OperationalError: database is locked
-