chore: add PHPDoc comments to types and request classes

Enrich the PHP SDK with class-level and property-level PHPDoc
documentation across a wide range of types and request DTOs. Every
added comment is purely descriptive — no public signatures, method
bodies, or serialization behavior were changed.

Key changes:
- Add class-level PHPDoc summaries to ~40 types (e.g., `Assistant`,
  `Call`, `CompliancePlan`, `Edge`, `BackoffPlan`, `ConversationNode`)
- Add property-level `@var` descriptions to previously undocumented
  fields (e.g., `firstMessageInterruptionsEnabled`, `compliancePlan`,
  `keypadInputPlan`, `baseDelaySeconds`, `excludedStatusCodes`)
- Reword several existing `@var` descriptions for clarity without
  changing any runtime behavior

🌿 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 renamed to `CartesiaSpeedControlZero`. Update any references to this enum in your code to use the new name.