Document Versioning3 test flows

[Shivs11]

What changed

• Add numbered Flow: comments to the three explicit pinned child override tests.
• Record the tests/versioning_3_test.go flow-comment convention in AGENTS.md.

Why

These functional tests share setup through a helper, so documenting each scenario's setup, action, and assertions makes the individual test cases easier to review and maintain.

Validation

• git diff --check
• Adversarial review accepted with no findings
• Tests not run because the change only adds comments and repository guidance

---

Note

Low Risk
Comment-only and contributor-guidance changes; no runtime or test logic modified.

Overview
Documents how reviewers should read shared-helper Versioning3 functional tests and codifies that convention for future changes.

AGENTS.md now requires new or updated tests in tests/versioning_3_test.go to include a concise numbered Flow: comment covering setup, action, and assertions.

Three explicit pinned child-override scenarios gain Flow: blocks: same task queue (pinned parent), cross task queue, and explicit override over an AutoUpgrade parent—each spelling out version registration, parent start behavior, child override, and expected routing/metadata.

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[Shivs11]

@stephanos -- personally, i think this should be included for this suite since tests here are quite convoluted/difficult to follow when doing a first pass

an example of a "flow" can be seen in the previous file -- i am tempted to add more suites for this rule, or just make this in general but curious to hear what you think?

[stephanos]

No objections; my only suggestion would be to be a little more precise. Unless you've tested it and now it works, I'm not sure these instructions will give you these flow comments you're adding here. For instance, it doesn't mention that they are numbered.