feat(types)!: default serialize_as_any=True in AdCPBaseModel.model_dump

[bokelley]

Summary

• Defaults serialize_as_any=True in AdCPBaseModel.model_dump() and model_dump_json(), symmetric with the existing exclude_none=True default.
• Subclass @model_serializer overrides now fire automatically through base-typed parent fields. Field(exclude=True) remains the wire-isolation contract and continues to suppress internal fields at every nesting depth.
• Removes the per-response-type parent model_dump boilerplate that adopters previously needed to write (the salesagent SDK alone has ~14 such overrides on top of 58 Field(exclude=True) markers).

Closes #615.

Why this and not the docs-only fix in #630

#630 documented the workaround: "you must remember serialize_as_any=True at every call site." That's the strongest argument for making it a default — if the kwarg is worth documenting as a footgun, it's worth flipping. The change is a one-line addition that's perfectly symmetric with exclude_none=True and eliminates the need for the workaround.

Behavior change

Subclasses that add fields without Field(exclude=True) previously had those fields silently dropped by Pydantic's declared-schema firewall when nested under a base-typed parent. Under this PR they appear in model_dump() output. The migration is mechanical: audit each subclass, mark internal fields with Field(exclude=True). Tagged feat! so release-please bumps the major version and the CHANGELOG flags the change clearly. Adopters who want the prior behavior at a specific call site can pass serialize_as_any=False explicitly.

docs/extending-types.md is updated to reflect the new default and call out the audit step for adopters with non-excluded subclass fields.

Test plan

• tests/test_serialize_as_any_default.py — 8 new tests pinning: subclass @model_serializer fires on singular and list fields, model_dump_json carries the same default, Field(exclude=True) continues to suppress internal fields, the new wire-leak path is observable (so adopters who relied on the firewall surface a failing test), explicit serialize_as_any=False opt-out works, and the two defaults (exclude_none, serialize_as_any) are independent.
• Full suite green locally — 3910 passed, no existing test relied on the firewall behavior.
• mypy clean on the touched files; mypy --strict tests/type_checks/ (the suite added in feat(testing): adopter type-checking test suite with zero-ignore contract #634) still clean.
• Pre-commit hooks pass.

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