docs(agents): guideline for avoiding test duplication

[LukaszRozmej]

Changes

• AGENTS.md "Coding guidelines and style" — new bullet on avoiding code duplication, especially in tests:
  • parameterize with [TestCase(...)] / [TestCaseSource(...)] when tests differ only by inputs/expected values
  • extract helper methods or types (builders, shared static helpers, base fixtures) when only parts of tests are similar
  • cross-link to test-infrastructure.md for the concrete patterns
• .agents/rules/test-infrastructure.md "Test guidelines" — extend the existing TestCase/TestCaseSource rules with concrete patterns for partial duplication:
  • shared arrange/build → builders (Build.A.Block..., TestItem.*) or private helpers
  • shared assertions → helper methods so each test asserts in one line
  • shared scenarios across files → base fixture, shared static helper, or fixture-level [SetUp]
  • guard against helpers that hide test-relevant behavior
  • note when to prefer [TestCaseSource] over [TestCase] (non-constant data, named scenarios, grows beyond a handful)

Complements PR #11544 which added the comments / XML-doc guidance. Same kind of cross-cutting agent rule, kept in a separate PR for clean review.

Types of changes

• Documentation update

Testing

Requires testing

• No

Documentation

Requires documentation update

• No

Requires explanation in Release Notes

• No