feat: send a LocalAI User-Agent on registry pulls (#10434)

LocalAI pulls models from OCI registries (via go-containerregistry), the
Ollama registry, and OCI blob stores (via oras), but every request went
out with the underlying library's generic User-Agent, so registry
operators had no way to attribute traffic to LocalAI.

Add an oci.UserAgent() helper that returns "LocalAI" (or
"LocalAI/<version>" when the binary is built with a version stamp via
internal.Version) and wire it into all three pull paths:

- pkg/oci/image.go: remote.WithUserAgent on the go-containerregistry
  image and digest requests
- pkg/oci/ollama.go: a User-Agent header on the Ollama manifest request
- pkg/oci/blob.go: a LocalAI User-Agent on the oras blob client. This
  mirrors oras' auth.DefaultClient (same retry.DefaultClient policy);
  only the advertised User-Agent changes.

Implements #6258.


Assisted-by: Claude:claude-opus-4-8 golangci-lint

Signed-off-by: Vijay Sai <vijaysaijnv@gmail.com>

docs/content/getting-started/models.md

@@ -131,6 +131,10 @@ local-ai run ollama://gemma:2b
 local-ai run oci://localai/phi-2:latest
 ```
 
+{{% notice note %}}
+When pulling models from Ollama or OCI registries, LocalAI identifies itself with a `LocalAI/<version>` `User-Agent` header so registry operators can attribute usage to LocalAI.
+{{% /notice %}}
+
 ### Run Models via URI
 
 To run models via URI, specify a URI to a model file or a configuration file when starting LocalAI. Valid syntax includes: