fix: resolve linter issues (gci import grouping, gofumpt, govet shadow)

- Separate third-party imports from local module imports (gci)
- Fix byte slice literal formatting (gofumpt)
- Rename shadowed err variable to ftErr (govet)
- Remove trailing blank lines in test files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/providers/openai_compat/provider_test.go

@@ -513,4 +513,3 @@ func TestSerializeMessages_StripsSystemParts(t *testing.T) {
 		t.Fatal("system_parts should not appear in serialized output")
 	}
 }
-