Add missing plugin configuration schema for LanguageModelFailurePlugin

[Copilot]

The LanguageModelFailurePlugin was added in PR #1314 but was missing its configuration schema. This PR adds the JSON schema file to ensure proper extensibility and documentation for plugin configuration.

Changes

• Added schemas/v1.0.0/languagemodelfailureplugin.schema.json with proper JSON Schema Draft 2020-12 format
• Schema includes the failures property as an optional array of strings matching the LanguageModelFailureConfiguration.Failures property
• Comprehensive description documents all 15 default failure types available in the plugin
• Follows established schema patterns in the repository with additionalProperties: false validation

Schema Properties

• failures: Optional array of failure type strings to simulate in language model responses
• Default behavior: When not specified, uses built-in failure types including AmbiguityVagueness, BiasStereotyping, CircularReasoning, ContradictoryInformation, FailureDisclaimHedge, FailureFollowInstructions, Hallucination, IncorrectFormatStyle, Misinterpretation, OutdatedInformation, OverSpecification, OverconfidenceUncertainty, Overgeneralization, OverreliancePriorConversation, and PlausibleIncorrect

Example Configuration

{
  "$schema": "https://github.com/raw/dotnet/dev-proxy/main/schemas/v1.0.0/languagemodelfailureplugin.schema.json",
  "failures": [
    "Hallucination",
    "BiasStereotyping", 
    "Misinterpretation"
  ]
}

Fixes #1320.

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