fix(site): make Base URL placeholder provider-aware (#23350)

The Base URL field in the provider config form always showed
`https://api.example.com/v1` as its placeholder, regardless of the
selected provider. This was confusing — I added `/v1` to my Anthropic
base URL because the placeholder suggested it, but the Anthropic SDK
already prefixes its request paths with `v1/`, so this doubled it up
and broke requests.

The placeholder is now provider-aware:

- **Anthropic, Bedrock, Google** → `https://api.example.com`
- **OpenAI-family providers** (openai, openai-compat, openrouter,
vercel, azure) → `https://api.example.com/v1`

site/src/pages/AgentsPage/ChatModelAdminPanel/ProviderForm.tsx

@@ -49,6 +49,13 @@ export const ProviderForm: FC<ProviderFormProps> = ({
 	const apiKeyInputId = useId();
 	const baseURLInputId = useId();
 
+	// Providers backed by the OpenAI SDK expect /v1 in the base
+	// URL, while others (e.g. Anthropic) do not.
+	const baseURLPlaceholder =
+		provider === "anthropic" || provider === "bedrock" || provider === "google"
+			? "https://api.example.com"
+			: "https://api.example.com/v1";
+
 	// Initial values are snapshotted when the provider config changes
 	// so we can detect dirty state.
 	const [initialValues] = useState(() => ({
@@ -241,7 +248,7 @@ export const ProviderForm: FC<ProviderFormProps> = ({
 								id={baseURLInputId}
 								name="provider_base_url"
 								className="h-9 text-[13px]"
-								placeholder="https://api.example.com/v1"
+								placeholder={baseURLPlaceholder}
 								autoComplete="off"
 								value={baseURLValue}
 								onChange={(e) => setBaseURLValue(e.target.value)}