Wire base_url + protocol through to protocol clients

[Kamilbenkirane]

Problem

Users cannot point celeste at OpenAI-compatible or Anthropic-compatible APIs (MiniMax, vLLM, Ollama, LocalAI, etc.). The base_url parameter exists in method signatures (generate(), analyze(), stream.generate()) but never reaches _build_url() — it's dead plumbing.

Surfaced by #200.

Root Cause

_build_url() in both protocol clients (OpenResponsesClient, ChatCompletionsClient) reads from self._default_base_url (a ClassVar). The base_url parameter flows through namespace/modality layers but:

1. _predict() doesn't have an explicit base_url param — it falls into **parameters
2. Protocol _make_request() / _make_stream_request() don't have base_url as a named param — it falls into **parameters and is ignored
3. _build_url() never sees it

Proposed Solution

Add protocol= + base_url= to create_client() and domain namespace methods:

# Anthropic-compatible (chatcompletions protocol)
await celeste.text.generate(
    "Explain quantum computing",
    model="MiniMax-M2.5",
    protocol="chatcompletions",
    base_url="https://api.minimax.io/anthropic",
    api_key="your-key",
)

# OpenAI-compatible (openresponses protocol)
await celeste.text.generate(
    "Hello",
    model="lfm2:latest",
    protocol="openresponses",
    base_url="http://localhost:11434",
)

Implementation

1. Add base_url to create_client() — store as instance attribute on the client
2. _build_url() uses self._base_url or self._default_base_url
3. Add protocol= as a way to select wire format without implying a provider identity
4. Relax model resolution when base_url is present — model + protocol + base_url should not hard-fail on unregistered models
5. Remove base_url from per-call method signatures (belongs on the client, not per-call)
6. Remove Ollama's custom base_url handling in OllamaGenerateClient._make_request() — use the generic mechanism

Protocol mapping

Protocol	Providers using it
openresponses	OpenAI, xAI, Ollama
chatcompletions	Anthropic, DeepSeek, Groq, HuggingFace, Mistral, Moonshot, Cerebras
Custom	Google (GenerateContent), Cohere