feat: add service_tier support for priority processing

[double-di]

Summary

Add per-request service_tier parameter to all SDK endpoints, enabling users to opt into priority processing.

Usage

from xai_sdk import Client
from xai_sdk.chat import user

client = Client()

# Set priority on a chat request
chat = client.chat.create(model="grok-4.3", service_tier="priority")
chat.append(user("Hello!"))
response = chat.sample()
print(response.service_tier)  # 'priority' or 'default'

# Also works on image and video endpoints
image = client.image.sample(
    prompt="A sunset over mountains",
    model="grok-imagine-image",
    service_tier="priority",
)

Changes

New type

• ServiceTier: Literal["auto", "default", "priority"]

Chat

• service_tier parameter on client.chat.create()
• Response.service_tier property — returns the tier actually used for the request

Image (async + sync)

• service_tier parameter on sample(), sample_batch(), prepare()

Video (async + sync)

• service_tier parameter on generate(), extend(), start(), extend_start(), prepare(), prepare_extension()

Proto bindings

• Updated usage_pb2 with ServiceTier enum
• Updated chat_pb2, image_pb2, video_pb2 with service_tier fields

All new parameters are optional with None default — fully backward compatible.