docs(test): expand test guidelines with best practices for mocking, filesystem, and wizard testing

[roger-zhangg]

Problem

The existing test guidelines in docs/CODE_GUIDELINES.md were minimal and didn't cover common testing patterns and anti-patterns discovered during test reviews.

Solution

Expanded the "Test guidelines" section in docs/CODE_GUIDELINES.md with practical guidance on:

• Real filesystem operations: Use TestFolder and testFixtures instead of stubbing fs methods
• Module imports: Import at top level instead of dynamic require() for stubbing
• Minimum mocks: Table showing what to mock vs. what not to mock
• Dependency injection: Design modules to accept context objects for testability
• URI methods: Use vscode.Uri.file() for filesystem paths
• PrompterTester: How to test wizard flows with QuickPicks and InputBoxes
• API parameter verification: Reference to mocked server pattern for verifying request payloads

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• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.