contrib.blacksheep.register_tortoise breaks under ASGI: "No TortoiseContext is currently active" on every DB query (missing _enable_global_fallback=True)

[dgavrilov]

Describe the bug

With Tortoise ORM 1.1.7, tortoise.contrib.blacksheep.register_tortoise sets up
the ORM in the app's on_start (ASGI lifespan) hook, but any DB query inside a
request handler raises:

RuntimeError: No TortoiseContext is currently active. Use 'async with
TortoiseContext() as ctx:' to create one, or call Tortoise.init() for global state.

The lifespan hook runs in a different asyncio task than request handlers. Since
1.x stores connection state in a contextvars.ContextVar (_current_context),
the context entered during startup is not visible in handler tasks, and no global
fallback is set — so require_context() fails.

The FastAPI integration does not have this problem because
RegisterTortoise defaults _enable_global_fallback=True
(tortoise/contrib/fastapi/__init__.py, _enable_global_fallback: bool = True).
The blacksheep integration calls Tortoise.init(...) without that flag, so
the global fallback context described in tortoise/context.py (lines ~48-52) is
never enabled.

To Reproduce

import asyncio
from tortoise import Tortoise

CONFIG = {
    "connections": {"default": "sqlite://:memory:"},
    "apps": {"models": {"models": ["__main__"], "default_connection": "default"}},
}

from tortoise import fields, models
class User(models.Model):
    id = fields.IntField(pk=True)

async def run(enable_fallback: bool) -> str:
    async def init_task():
        await Tortoise.init(config=CONFIG, _enable_global_fallback=enable_fallback)
        await Tortoise.generate_schemas()
    await asyncio.create_task(init_task())          # like the lifespan task

    async def query_task():
        await User.all().count()                    # like a request handler
        return "ok"
    try:
        return await asyncio.create_task(query_task())
    except RuntimeError as exc:
        return f"RuntimeError: {exc}"
    finally:
        await Tortoise.close_connections()

async def main():
    print("without:", await run(False))
    print("with:   ", await run(True))

asyncio.run(main())

Output:

without: RuntimeError: No TortoiseContext is currently active. ...
with:    ok

A full repro with an actual BlackSheep app + register_tortoise shows the same
RuntimeError on the first request that touches the DB.

Expected behavior

register_tortoise should make the ORM usable from request handlers out of the
box, the same as the FastAPI integration.

Additional context

Pass _enable_global_fallback=True in the blacksheep integration's
Tortoise.init(...) call (matching contrib.fastapi.RegisterTortoise).