Add Sonnet 5 prompt setting

[bhavyaus]

Adds an experiment-gated Sonnet 5 system prompt and model-routing coverage.

Tests: Copilot typecheck, lint, and agent prompt tests.

[Copilot]

Pull request overview

Adds an experiment-gated system prompt customization and routing for Claude Sonnet 5.

Changes:

• Registers the Sonnet 5 experimental setting.
• Adds Sonnet 5 prompt instructions and resolver routing.
• Tests enabled, disabled, and non-Sonnet behavior.

Show a summary per file

File	Description
configurationService.ts	Defines the experiment-backed setting.
anthropicPrompts.spec.tsx	Tests prompt selection and isolation.
anthropicPrompts.tsx	Implements and routes the Sonnet 5 prompt.
package.nls.json	Adds the localized setting description.
package.json	Contributes the experimental setting.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Balanced

[Copilot]

Review details

Suppressed comments (1)

extensions/copilot/src/extension/prompts/node/agent/anthropicPrompts.tsx:583

• The new experiment-enabled routing branch is not exercised by the agent prompt snapshots: test/agentPrompt.spec.tsx:34-50 has no Sonnet 5 family, and the flag defaults to false. Please add coverage that enables ClaudeSonnet5PromptEnabled for a Sonnet 5 endpoint and verifies the new prompt (plus the disabled fallback), otherwise model matching or prompt rendering can regress unnoticed.

		if (this.isSonnet5(endpoint) && this.configurationService.getExperimentBasedConfig(ConfigKey.ClaudeSonnet5PromptEnabled, this.experimentationService)) {
			return ClaudeSonnet5Prompt;

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Balanced