docs: document Bedrock mantle protocol (#27296)

Implements:
https://linear.app/codercom/issue/AIGOV-518/add-docs-for-bedrock-mantle
Follow-up to: https://github.com/coder/coder/pull/26745

docs/ai-coder/ai-gateway/providers.md

@@ -86,10 +86,42 @@ to have restricted permissions at the time of writing (June 2026).
 Bedrock providers serve Anthropic models hosted on AWS and authenticate
 with AWS credentials rather than a registered API key. Configure:
 
-- A **region** (or a full base URL when routing through a proxy or a
-  non-standard endpoint that does not follow the
-  `https://bedrock-runtime.<region>.amazonaws.com` format).
-- The **model** and **small fast model** identifiers.
+- A **protocol**, either `InvokeModel` or `Mantle`. It determines the
+  endpoint format and which of the fields below apply. See
+  [InvokeModel](#invokemodel) and [Mantle](#mantle).
+- An **endpoint** in the format the protocol requires:
+  `https://bedrock-runtime.<region>.amazonaws.com` for InvokeModel or
+  `https://bedrock-mantle.<region>.api.aws/anthropic` for Mantle. The AWS
+  region is read from the endpoint host.
+- The **model** and **small fast model** identifiers, for InvokeModel
+  only. Mantle providers do not set them; the client chooses the model on
+  each request and AI Gateway forwards it upstream.
+
+#### InvokeModel
+
+The legacy Bedrock runtime API. AI Gateway translates each request into
+Bedrock's InvokeModel format and sends it to
+`https://bedrock-runtime.<region>.amazonaws.com`. Still supported, but
+Mantle is recommended for new deployments.
+
+#### Mantle
+
+The newer Anthropic-compatible Bedrock endpoint, recommended by AWS for new
+deployments. AI Gateway serves Anthropic models through the native Messages
+API, forwarding the request body **unchanged** and only applying AWS SigV4
+signing with the provider's base identity.
+
+To route Claude Code through a Mantle provider, run it in mantle mode with
+client-side signing disabled so the gateway signs centrally:
+
+```sh
+export CLAUDE_CODE_USE_MANTLE=1
+export CLAUDE_CODE_SKIP_MANTLE_AUTH=1
+export ANTHROPIC_BEDROCK_MANTLE_BASE_URL="<your-deployment-url>/api/v2/ai-gateway/<provider-name>"
+export ANTHROPIC_AUTH_TOKEN="<your-coder-api-token>"
+```
+
+#### AWS credentials
 
 Do not attach API keys to a Bedrock provider.
 

site/src/pages/AISettingsPage/ProvidersPage/components/ProviderForm.tsx

@@ -573,9 +573,7 @@ export const ProviderForm: FC<ProviderFormProps> = ({
 									<SelectValue />
 								</SelectTrigger>
 								<SelectContent>
-									<SelectItem value="invoke-model">
-										InvokeModel (default)
-									</SelectItem>
+									<SelectItem value="invoke-model">InvokeModel</SelectItem>
 									<SelectItem value="mantle">Mantle</SelectItem>
 								</SelectContent>
 							</Select>