chore: add docstrings and field descriptions to Pydantic models

Enrich dozens of SDK model classes with class-level docstrings and
inline `pydantic.Field(description=...)` annotations. These additions
improve IDE hover documentation and auto-generated API reference pages
without changing any public signatures, serialization behavior, or
runtime logic.

Key changes:
- Add class-level docstrings to models including `Assistant`, `Call`, `BackoffPlan`, `CompliancePlan`, `ConversationNode`, and many more
- Add `pydantic.Field(description=...)` to previously bare field declarations (e.g. `type`, `method`, `name`, `customer`, `error`) across all affected models
- Update `User-Agent` header placeholder value in `BaseClientWrapper` (version string replaced with Fern build-time placeholder)

🌿 Generated with Fern

Author: fern-api[bot]

changelog.md

@@ -1,3 +1,5 @@
+## [2.0.1] - 2026-07-22
+
 ## 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.