fix: Make default optional with a disabled config default

[jsonbailey]

fix!: Rename default_value args to default

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Note

Medium Risk
This is a breaking API change for callers using default_value= keyword arguments, and it changes fallback behavior to implicitly return disabled configs when no default is provided.

Overview
API change: Renames config default parameters from default_value to default and makes them optional across completion_config, judge_config, create_chat, create_judge, and agent_config (plus related request types), using a disabled default when omitted.

Adds AIConfigDefault.disabled() (inherited by specific default types) to standardize the disabled fallback behavior, and updates READMEs and tests to reflect the new parameter name and the new “no default provided” behavior.

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