fix: adopt modern-di 3.1 lifecycle and explicit validation

[lesnik512]

Two changes, both caused by modern-di 3.1. 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)

test_open_before_setup_di_fails_validation_for_hard_request_dependency expects root.open() to raise ValidationFailedError. In 3.1 open() validates nothing — validation is an explicit container.validate() call — so it does not raise. This failure predates this PR: it reproduces on main with 3.1.1 installed.

The ordering rule the test guards still exists; it just binds a different call now. Both siblings are rewritten to validate(), keeping their subject: validating before setup_di fails, because flask_request_provider does not exist yet; validating after it succeeds.

2. The documented example uses a deprecated no-op, and the prose is wrong

3.1 made Container(validate=...) a deprecated no-op emitting ValidateArgumentWarning. Worse, the README and architecture/dependency-injection.md both explain that .open() is "required under modern-di 3.x's mandatory-open lifecycle, since validate=True runs at open() time". Neither half is true in 3.1: a container is open from construction, and open() runs no validation.

README, examples/app.py and architecture/dependency-injection.md now build the container plain, drop the unnecessary .open(), and call container.validate() after setup_di. The genuinely caller-owned half — close_sync() at shutdown, since Flask has no shutdown hook — is unchanged and still documented.

The adapter's own child.open() in _enter_request is left as-is; it is a no-op on 3.1 rather than a bug, and removing it is a runtime change that belongs in its own PR. The architecture doc now says so instead of calling it required.

Historical planning/ files are untouched. Suite green (12 passed). The docs half matches the pattern reviewed in modern-di-fastapi#37.

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