[fern-autoversion] feat!: migrate module path from v2 to non-versioned import path

The Go module path has been changed from `github.com/VapiAI/server-sdk-go/v2`
to `github.com/VapiAI/server-sdk-go` across all packages. This is a breaking
change for existing consumers who must update their import paths accordingly.

The User-Agent header has also been updated to reflect the new module path.

Key changes:
- All internal package imports updated from `.../v2/...` to `.../...` (e.g., `github.com/VapiAI/server-sdk-go/v2/core` → `github.com/VapiAI/server-sdk-go/core`)
- User-Agent header changed from `github.com/VapiAI/server-sdk-go/v2.0.0` to `github.com/VapiAI/server-sdk-go/0.0.0-fern-placeholder`
- `.fernignore` updated to exclude replay and gitattributes files

🌿 Generated with Fern

Author: fern-api[bot]

.fern/metadata.json

@@ -6,5 +6,5 @@
     "union": "v1"
   },
   "originGitCommit": "d43c62976eed622763119de871f9302955fd21a8",
-  "sdkVersion": "v0.0.0-fern-placeholder"
+  "sdkVersion": "v3.0.0"
 }