docs(test): redefine unit-vs-e2e policy for a6

[guoqqqi]

Refs #19

Goal

Update repository guidance so the new testing rules are explicit and reviewable:

• no new mocked APISIX Admin API command tests
• command behavior coverage must go through real APISIX E2E
• unit tests are reserved for pure logic and self-contained helpers
• Ginkgo/Gomega is the target framework for E2E going forward

Scope

• rewrite docs/testing-strategy.md
• update AGENTS.md and any contributor guidance that still recommends httpmock for command-layer tests
• document the allowed unit-test boundary and the disallowed mocked-command-test pattern
• document migration policy for existing tests: phased removal, not one-shot deletion

Acceptance Criteria

• repository docs no longer instruct authors to add mocked Admin API tests for pkg/cmd/**
• docs clearly separate unit and e2e responsibilities
• docs mention Ginkgo/Gomega as the standard E2E direction
• reviewers can point to a single source of truth for future test PRs

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• test: refactor automation strategy around real APISIX E2E and Ginkgo #19

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