Replace Literal model type with string in Python SessionConfig

[Copilot]

Python SDK's SessionConfig.model used a hardcoded Literal type that becomes stale as model availability changes. Node.js SDK uses string with runtime validation by the CLI server.

Changes

• python/copilot/types.py: Changed model field from Literal["gpt-5", "claude-sonnet-4", "claude-sonnet-4.5", "claude-haiku-4.5"] to str
• Added inline documentation pointing users to list_models() for discovering available models

Impact

# Before: Only specific models allowed by type system
config: SessionConfig = {"model": "gpt-5"}  # OK
config: SessionConfig = {"model": "gpt-4"}  # Type error

# After: Any string accepted, validated at runtime by CLI
config: SessionConfig = {"model": "gpt-5"}   # OK
config: SessionConfig = {"model": "gpt-4"}   # OK
config: SessionConfig = {"model": "new-model"}  # OK

Aligns Python SDK with Node.js SDK's flexible approach. Model validation occurs at session creation time by the CLI server.

Original prompt

SessionConfig in python has model: Literal["gpt-5", "claude-sonnet-4", "claude-sonnet-4.5", "claude-haiku-4.5"] but that's not the right approach, this field should be an optional string, and then we should do runtime validation based on listModels, similar to how the nodejs sdk does it.

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[Copilot]

Pull request overview

This PR updates the Python SDK's SessionConfig.model type to match the Node.js SDK's flexible approach, allowing any string value instead of hardcoded model names that quickly become stale.

Changes:

• Changed model field type from Literal with specific model names to str
• Added inline documentation directing users to list_models() for discovering available models

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[stomde]

Great 😁😁 okay

[stomde]

Shitty coins