sdk: add the @start decorator

[eunomie]

Adds @start to mark a function as a service for dagger start (dagger/dagger#14218). The engine part is dagger/dagger#14328.

• @start sets the same flag @up did, so registration still calls Function.withUp. The wire name does not change, so this works on engines that predate start.
• @up keeps working. It is deprecated with typing_extensions.deprecated, which emits a DeprecationWarning that points at @start. Python filters that warning by default, so existing modules run unchanged.
• start is exported next to up in dagger.mod.__all__, so from dagger import start works.

This mirrors the sdk/python change in dagger/dagger#14328.

The with_up docstring in sdk/src/dagger/client/gen.py still says "dagger up". That file is generated from the engine schema and changes when the engine pin moves past dagger/dagger#14328.

Testing:

• uv run --frozen pytest -q tests/client tests/codegen tests/mod: new tests cover @start in both decorator orders, and @up warning while still marking a service.
• A module vendoring this library, run by the released v1.0.0-beta.14 engine, which predates start: dagger up -l lists both the @start and the @up function, and dagger up proof:web serves the @start service on the forwarded port.