chore: add docstrings and field descriptions to Pydantic models

Enrich dozens of Pydantic model classes across the SDK with class-level
docstrings and per-field `description` arguments. No public API surface,
method signatures, or runtime behavior has changed — all additions are
purely documentation metadata surfaced through `pydantic.Field()`.

Key changes:
- Add class-level docstrings to models such as `Assistant`, `Call`, `BackoffPlan`, `CompliancePlan`, `ConversationNode`, and many more
- Add `description=` to `pydantic.Field()` calls for previously undocumented fields (e.g. `firstMessageInterruptionsEnabled`, `compliancePlan`, `keypadInputPlan`, `baseDelaySeconds`, `excludedStatusCodes`)
- Update `User-Agent` header version placeholder to `0.0.0-fern-placeholder` ahead of release versioning

🌿 Generated with Fern

Author: fern-api[bot]

changelog.md

@@ -1,3 +1,5 @@
+## [2.0.1] - 2026-07-20
+
 ## 2.0.0 - 2026-06-24
 ### Breaking Changes
 * **`CartesiaExperimentalControlsSpeedZero`** has been removed and replaced by **`CartesiaSpeedControlZero`**. Update any imports or type annotations referencing `CartesiaExperimentalControlsSpeedZero` to use `CartesiaSpeedControlZero` instead.