fix: restore agent instruction guidance

[sunxiayi]

Commit Type

• feature - New functionality
• fix - Bug fix
• refactor - Code restructuring without behavior change
• perf - Performance improvement
• docs - Documentation update
• test - Test-related changes
• chore - Maintenance/tooling

Risk Level

• Low - Minor changes, limited scope
• Medium - Moderate changes, some user impact
• High - Major changes, significant user/system impact

What & Why

Restores the missing system-instruction guidance and the “Tips for writing agent instructions” documentation link in AgentInstructionEditor.

The guidance was removed from the shared editor in #8402, which caused the content reported in #8531 to disappear in both Standard and Consumption experiences. The restored content remains inside the existing !hideSystemInstructions branch so Foundry-managed scenarios that hide the system editor do not display unrelated guidance.

Fixes #8531.

Impact of Change

• Users: Agent instruction editors again show the explanatory copy and documentation link when system instructions are available.
• Developers: Adds focused regression coverage for the visible and hidden system-instruction states; no API changes.
• System: No dependency, architecture, or performance changes. Base localization keys are restored through the repository extraction workflow.

Test Plan

• Unit tests added/updated
• E2E tests added/updated
• Manual testing completed
• Tested in: local render of the real AgentInstructionEditor; pnpm --filter @microsoft/designer-ui test:lib (120 files, 1088 tests); pnpm --filter @microsoft/designer-ui build:lib; Biome; and tsc --noEmit

Contributors

• sunxiayi
• Codex assisted with root-cause analysis, implementation, automated test preparation, and local visual verification.

Screenshots/Videos

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[Copilot]

🟡 Changes recommended

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Pull request overview

Restores localized guidance and documentation for agent system instructions while preserving Foundry-managed hiding behavior.

Changes:

• Restores explanatory text and documentation link.
• Adds localization resources.
• Adds visible/hidden-state regression tests.

File summaries

File	Description
Localize/lang/strings.json	Adds guidance localization strings.
libs/designer-ui/src/lib/agentinstruction/index.tsx	Renders guidance when system instructions are visible.
libs/designer-ui/src/lib/agentinstruction/__test__/agentinstruction.spec.tsx	Tests guidance visibility and link attributes.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Balanced

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