fix: isolate model imports from dependency runtime

[marlon-costa-dc]

Root cause\n\nImporting the public model facade traversed collection metadata normalization through the composite runtime facade. That initialized dependency-injector wiring and FastAPI/OpenAPI during test collection. Under host commit pressure, Pydantic schema construction then failed with MemoryError before tests could run.\n\n## Change\n\n- make the internal runtime package exports lazy\n- route collection metadata normalization to its leaf owner\n- prove that resolving flext_core.m.StrictModel loads neither dependency_injector nor fastapi\n\n## Evidence so far\n\n- RED: make test WHAT=all FILE=tests/unit/test_lazy_exports.py — new regression failed and listed the unintended modules\n- GREEN: same command — 1 passed, 23 deselected\n- make fmt — 524 files already formatted\n\nWIP: consumer and full native gates still running.

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Summary by cubic

Isolates model imports from the dependency runtime and types the facade's field/private-attr helpers so test collection no longer initializes dependency_injector or FastAPI/OpenAPI (which failed with MemoryError under memory pressure) and PrivateAttr supports default, default_factory, and per-instance state.

• Makes the internal runtime package exports lazy.
• Routes collection metadata normalization to the leaf _runtime._metadata owner instead of the composite runtime facade.
• Types facade-exported PrivateAttr and reuses it in the utilities facade so both m.PrivateAttr and u.PrivateAttr support default, default_factory, and per-instance state.
• Types immutable mapping defaults so generated schemas retain concrete value types.
• Replaces lambdas with functools.partial and typed callback helpers in handlers, config, and mapper to keep type contracts explicit.
• Adds regression tests for the lazy import path and private-attr instance semantics.

^{Written for commit 187453f. Summary will update on new commits.}

[marlon-costa-dc]

Root-cause evidence: the new public regression first failed by listing dependency_injector plus 38 FastAPI/OpenAPI modules after resolving flext_core.m.StrictModel, then passed after the owner fix. The original flext-infra consumer also completed 21/21 in tests/unit/codegen/test_codegen_ci_matrix.py without MemoryError. Operator approval was provided in the active rollout session; full remote gates remain required.

[sonarqubecloud]

Quality Gate passed

Issues
15 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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