docs: require Cache Components compatibility for new e2e tests (#101222)
### What?
Add guidance in `test/AGENTS.md` requiring new e2e tests to support
Cache Components both enabled and disabled.
### Why?
The test infrastructure runs both variants, and Cache Components can be
enabled even when the fixture does not declare the option in
`next.config`. New tests should follow those rules rather than
accidentally relying on the flag being disabled.
### How?
Keep the policy in the test-specific agent guide. Require compatible
fixtures and meaningful assertions in both variants, reserving explained
test gates for genuinely unsupported features such as Edge Runtime.
Refer to the existing resolved-config gate mechanism and distinguish
skipping from expected-failure behavior.
### Verification
- Prettier check passed for `test/AGENTS.md`.
- Whitespace checks passed.
- No runtime tests run: agent-guidance-only change.
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Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>