fix: correct session.chat() return type to match server response

[illera88]

Summary

• Adds SessionChatResponse type with info: Message and parts: List[Part] fields matching the actual server response.
• Updates both sync and async chat() methods to return SessionChatResponse instead of bare AssistantMessage.

Problem

The OpenCode server's POST /session/{id}/message endpoint returns a wrapper:

{
  "info": { "role": "assistant", "id": "msg_...", "tokens": {...}, ... },
  "parts": [
    { "type": "step-start", ... },
    { "type": "text", "text": "Hello!", ... },
    { "type": "step-finish", ... }
  ]
}

But session.chat() was typed to cast this entire response to AssistantMessage, which only expects the fields inside info. Pydantic silently ignores the unknown top-level keys (info, parts) and produces a hollow object where id, role, tokens, etc. are all None.

Reproduction

from opencode_ai import Opencode

client = Opencode(base_url="http://127.0.0.1:4096")
session = client.session.create(extra_body={})

resp = client.session.chat(
    session.id,
    model_id="claude-sonnet-4-20250514",
    provider_id="anthropic",
    parts=[{"type": "text", "text": "Hello"}],
)

print(resp.id)    # None (should be "msg_...")
print(resp.role)  # None (should be "assistant")

Fix

Added SessionChatResponse (same shape as SessionMessagesResponseItem):

class SessionChatResponse(BaseModel):
    info: Message
    parts: List[Part]

After this fix:

resp = client.session.chat(session.id, ...)
print(resp.info.role)     # "assistant"
print(resp.info.id)       # "msg_..."
print(resp.info.tokens)   # Tokens(input=3, output=9, ...)
for part in resp.parts:
    if isinstance(part, TextPart):
        print(part.text)  # "Hello!"