chore: add conservative testing guidelines to AGENTS.md

[samsja]

Summary

• Add guidelines to AGENTS.md discouraging adding new tests unless explicitly requested or clearly necessary
• Editing existing tests is fine; adding new ones should be the exception
• Only test code with clear, isolated logic (pure functions, abstract classes, data transformations) — not runtime-level code that requires heavy mocking

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Note

Low Risk
Low risk: documentation-only changes with no impact on runtime behavior or production systems.

Overview
Adds more conservative testing guidelines to AGENTS.md, emphasizing that new tests should only be added when explicitly requested or clearly necessary.

Clarifies what to test by encouraging tests for isolated logic (e.g., pure functions and data transforms) and discouraging tests that require heavy mocking or exercise framework/runtime glue.

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