fix(tests): adopt modern-di 3.1 lifecycle, drop deprecated validate=

[lesnik512]

Two changes, both caused by modern-di 3.1 and both needed for this package to be correct against it. This package pins modern-di>=3,<4, so users and CI pick 3.1 up on a routine upgrade.

1. The suite is already red on 3.1 (fixed here)

modern-di 3.1 makes a container open from construction — 3.0 built them closed and required an explicit open(). Assertions written against the old contract now fail. These failures predate this PR: they reproduce on main with 3.1.1 installed.

The fix keeps each test's actual subject. Where the assertion was only a precondition, it now states the 3.1 one. Where the subject was the lifecycle transition, the container is closed first so the transition stays observable — otherwise the assertion would pass without the signal doing anything.

2. The documented example uses a deprecated no-op

3.1 also made Container(validate=...) a deprecated no-op that emits ValidateArgumentWarning, moving validation to an explicit container.validate() call:

• validate=True emits a DeprecationWarning on every construction, from copy-pasted official documentation. A suite following modern-di's own readiness recipe (filterwarnings = ["error::DeprecationWarning"]) fails on it.
• More quietly: it was the explicit opt-in to graph validation. It now does nothing, so anyone following these docs believes they are validated and is not.

README and examples/ now build the container plain and call container.validate() after setup_di — the ordering the core docs prescribe, since setup_di registers this integration's providers. Where the example built the container inline, it is hoisted so it can be validated at all. Tests drop the argument only.

Historical planning/ files are left untouched. Suite green after both changes. The docs half matches the pattern reviewed in modern-di-fastapi#37.

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