feat(types): provide version-scoped Pydantic extension base

[bokelley]

Summary

• add make_versioned_base(version, model_name) as a stable, public factory for subclassable version-pinned Pydantic models
• expose real protocol fields through model_fields while retaining public nested Pydantic annotations where available
• validate the serialized wire payload against the bundled pinned schema so excluded adopter fields never leak onto the wire
• preserve the existing version-scoped RootModel surface unchanged

Impact

Adopters can extend a historical protocol request or response with internal Pydantic fields declared with Field(exclude=True). The generated base remains pinned to the requested protocol release, rejects fields from newer releases, and emits the canonical bundled JSON Schema.

Validation

• uv run python -m pytest -q — 6,557 passed, 40 skipped, 9 deselected, 1 xfailed
• make lint
• make typecheck-all
• make validate-generated
• pre-commit hooks (black, ruff, mypy, adopter fixtures, bandit, repository checks)
• smoke-created all 497 version/model combinations across bundled 3.0, 3.1, and 3.2 schemas

Closes #1050

[aao-secretariat]

Medium: Nested fields reuse the current SDK annotation, so version pinning is top-level only. extra="forbid" plus the top-level schema reject newer-release fields at the root (tested in test_versioned_base_enforces_version_delta_fields), but a nested object is validated solely by the pinned schema's own additionalProperties. Published AdCP schemas default additionalProperties: true (DR-0009), and the reused current-version nested model (e.g. 3.2 CreativeFilters) will happily accept a 3.2-only sub-field; with exclude_unset=True it then rides onto a 3.1 wire. The PR's "rejects fields from newer releases" guarantee holds at the root, not inside nested structures — worth stating in the docstring and covering with a nested-delta test so the scope of the pin is explicit.

[aao-secretariat]

Ladon verdict: Approve

Approve — adds make_versioned_base(version, model_name), a cached, subclassable Pydantic factory that pins a model to a bundled protocol schema and validates the serialized (exclude=True-stripped) payload against the pinned schema before the wire.

What I checked:

• Fail-closed posture: extra="forbid" rejects undeclared top-level fields; Field(exclude=True) adopter fields stay out of schema validation and model_json_schema output (tested); final jsonschema pass is the wire authority.
• Change is confined to src/adcp/types/versioned.py plus its test file — no hand-edits to generated_poc/** or _generated.py, no import-layering violations, no forward-compat/discriminated-union regressions, no credential-in-ctx_metadata concerns, no disabled tests or blanket type-ignores.
• New public behavior ships with corresponding tests (tests/test_version_scoped_models.py).

Medium findings (1):

• src/adcp/types/versioned.py:455 — Version pinning is top-level only; nested objects track the current SDK model, so newer-release nested fields can pass when the pinned schema allows additionalProperties. This is a documented scope caveat, not a blocking defect.

Decision path: no critical/high findings; gated_paths is false (row 2 does not apply); high_risk false; the single medium is an LLM/scope caveat, not data-loss/schema/infra; no author team gate; fewer than three medium findings. Rows 1–8 do not fire → row 9 approve.

Medium findings

• src/adcp/types/versioned.py:455 — Version pinning is top-level only; nested fields track current SDK