refactor: single-source the connection-kind mapping off the providers

[lesnik512]

Completes candidate 1 from the architecture review (and finishes the single-sourcing #15 began). Behavior is unchanged.

What

build_di_container re-stated, in an isinstance ladder, the connection types and context keys that the two ContextProviders already carry — the connection-kind concept lived in two places.

modern-di 2.21.0 exposes ContextProvider.context_type (modern-python/modern-di#244, shipped 2.21.0), so the registered providers can now be the single source:

_CONNECTION_PROVIDERS = (fastapi_request_provider, fastapi_websocket_provider)

# setup_di
container.providers_registry.add_providers(*_CONNECTION_PROVIDERS)

# build_di_container
for provider in _CONNECTION_PROVIDERS:
    if isinstance(connection, provider.context_type):
        context[provider.context_type] = connection
        scope = provider.scope
        break

Adding a connection kind is now adding a provider to _CONNECTION_PROVIDERS; the dispatch and registration both flow from it.

Wins (in the review's terms)

• locality: connection-kind knowledge in one place
• leverage: one mapping drives both registration and dispatch
• interface unchanged; the isinstance ladder is absorbed

Changes

• modern_di_fastapi/main.py — the refactor above.
• pyproject.toml — floor bump modern-di>=2.21.0,<3 (needs the public context_type).
• architecture/container-lifecycle.md + architecture/glossary.md — promoted per the planning convention (the Scope mapping is now described as provider-sourced).

Verification

• just test-ci → 100% coverage, 7 passed (existing Request + WebSocket tests cover the dispatch).
• just lint-ci → ruff / ty / eof / check-planning all clean.

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