[fern-autoversion] feat!: migrate module path from /v2 to root import path

Remove the `/v2` suffix from all Go module import paths across the entire
SDK. Every sub-package (`analytics`, `assistants`, `calls`, `campaigns`,
`chats`, `client`, `eval`, `files`, `insight`, `observabilityscorecard`,
`option`, `phonenumbers`, `providerresources`, `sessions`, `squads`,
`structuredoutputs`, `tools`, and `core`/`internal`) now imports from
`github.com/VapiAI/server-sdk-go` instead of
`github.com/VapiAI/server-sdk-go/v2`. The `User-Agent` header value has
also been removed from `core/request_option.go`. Consumers must update
their `go.mod` and all import statements to use the new module path.

Key changes:
- All internal imports rewritten from `github.com/VapiAI/server-sdk-go/v2/...` to `github.com/VapiAI/server-sdk-go/...`
- `User-Agent` header removed from `RequestOptions.cloneHeader()`
- `.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": "a553b937d463f5d8b1904e74cfbbdcd69e72dde3",
-  "sdkVersion": "v0.0.0-fern-placeholder"
+  "sdkVersion": "v3.0.0"
 }