chore: remove User-Agent header from default request headers

Remove the hardcoded `User-Agent` header from the default headers sent
with every API request in VapiClient. Additionally, update .fernignore
to exclude replay and gitattributes files from Fern-managed changes.

Key changes:
- Remove `User-Agent: vapi/vapi/2.0.0` from default request headers in VapiClient
- Add `.fern/replay.lock`, `.fern/replay.yml`, and `.gitattributes` to .fernignore

🌿 Generated with Fern

Author: fern-api[bot]

changelog.md

@@ -1,3 +1,5 @@
+## [2.0.1] - 2026-07-15
+
 ## 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.