bug: Provider string reference isn't enough to resolve custom provider name.

[everettVT]

Describe the bug

When passing custom provider name as provider arg in Prompt, if you specify just the custom provider name in an ai function argument, resolve_provider misses the resolution and load_provider throws a ValueError: Provider 'OpenRouter' is not yet supported.

To Reproduce

# /// script
# description = "Prompt an OpenRouter model using a session"
# requires-python = ">=3.10, <3.13"
# dependencies = ["daft","openai","pydantic","python-dotenv"]
# ///
import os 
from dotenv import load_dotenv
import daft
from daft.ai.openai.provider import OpenAIProvider
from daft.functions.ai import prompt
from daft.session import Session



if __name__ == "__main__":
    # Load environment variables
    load_dotenv()

    # Create an OpenRouter provider
    openrouter_provider = OpenAIProvider(
        name="OpenRouter", 
        base_url="https://openrouter.ai/api/v1", 
        api_key=os.environ.get("OPENROUTER_API_KEY")
    )
    
    # Create a session and attach the provider
    sess = Session()
    sess.attach_provider(openrouter_provider)
    sess.set_provider("OpenRouter")

    # Create a dataframe with the quotes
    df = daft.from_pydict({
        "quote": [
            "I am going to be the king of the pirates!",
            "I'm going to be the next Hokage!",
        ],
    })
    
    # Use the prompt function to classify the quotes
    df = (
        df
        .with_column(
            "nemotron-response", 
            prompt(
                daft.col("quote"),
                system_message="You are an anime expert. Classify the anime based on the text and returns the name, character, and quote.",
                provider="OpenRouter", # but sess.get_provider("OpenRouter") works
                model="nvidia/nemotron-nano-9b-v2:free"
            )
        )
    )

    df.show(format="fancy", max_width=120)

Expected behavior

Should be a small update to _resolve_provider() in daft.functions.ai.init.py

Component(s)

Other

Additional context

THESE OTHER METHODS WORK

If connecting to OpenAI, users can naively specify "openai" to have an OpenAIProvider created and attached for them.

# /// script
# description = "Embed Video Frames from a Youtube Video"
# requires-python = ">=3.10, <3.13"
# dependencies = ["daft","openai","pydantic","python-dotenv"]
# ///

from dotenv import load_dotenv
import daft
from daft.functions.ai import prompt

if __name__ == "__main__":
    # Load environment variables
    load_dotenv()

    # Create a dataframe with the quotes
    df = daft.from_pydict({
        "quote": [
            "I am going to be the king of the pirates!",
            "I'm going to be the next Hokage!",
        ],
    })
    
    # Use the prompt function to classify the quotes
    df = (
        df
        .with_column(
            "response", 
            prompt(
                daft.col("quote"),
                system_message="You are an anime expert. Classify the anime based on the text and returns the name, character, and quote.",
                provider="openai", # Automatically creates an OpenAI provider 
                model="gpt-5-nano"
            )
        )
    )

    df.show(format="fancy", max_width=120)

Passing Custom Provider Object Directly (ie OpenRouter)

# /// script
# description = "Prompt an OpenRouter model"
# requires-python = ">=3.10, <3.13"
# dependencies = ["daft","openai","pydantic","python-dotenv"]
# ///
import os 
from dotenv import load_dotenv
import daft
from daft.ai.openai.provider import OpenAIProvider
from daft.functions.ai import prompt

if __name__ == "__main__":
    # Load environment variables
    load_dotenv()
    
    # Create an OpenRouter provider
    openrouter_provider = OpenAIProvider(
        name="OpenRouter", 
        base_url="https://openrouter.ai/api/v1", 
        api_key=os.environ.get("OPENROUTER_API_KEY")
    )

    # Create a dataframe with the quotes
    df = daft.from_pydict({
        "quote": [
            "I am going to be the king of the pirates!",
            "I'm going to be the next Hokage!",
        ],
    })
    
    # Use the prompt function to classify the quotes
    df = (
        df
        .with_column(
            "response", 
            prompt(
                daft.col("quote"),
                system_message="You are an anime expert. Classify the anime based on the text and returns the name, character, and quote.",
                provider=openrouter_provider,
                model="nvidia/nemotron-nano-9b-v2:free"
            )
        )
    )

    df.show(format="fancy", max_width=120)

[everettVT]

@rchowell