feat: support cross-account Bedrock AssumeRole in AI Bridge (#26527)

# Support IAM role assumption for AWS Bedrock in AI Bridge

## Summary

Implements
https://linear.app/codercom/issue/AIGOV-371/support-dynamic-bedrock-assumerole-across-aws-accounts-for-ai-gateway

A Bedrock provider can now be configured with an IAM role to assume.
Before calling Bedrock, the gateway assumes that role via STS and signs
requests with the resulting temporary credentials. Whether the role
lives in the same account or another one is entirely a matter of the
role's trust policy.

## Problem

Many organizations prohibit long-lived AWS access keys and expect
workloads to authenticate through assumed IAM roles instead. A common
case is an organization that runs Bedrock across several AWS accounts,
one per business unit, and needs each unit's usage billed to its own
account by assuming a role there. AI Bridge previously authenticated a
Bedrock provider only with static keys or the gateway's own ambient AWS
identity, which is shared by every provider, with no way to assume a
role. These deployments had no clean path.

## How it works

When a provider is configured with a role ARN, the gateway uses its base
identity to assume that role via STS and signs Bedrock requests with the
temporary credentials it returns. The base identity is whatever the AWS
default credential chain resolves, IRSA, EKS Pod Identity, EC2 Instance
Profile, or static keys.

Credentials are resolved once when the provider is set up and are then
cached and rotated, so individual requests are served from the cache
rather than triggering a new STS call. A deployment that needs several
roles configures several providers, each pointing at its own role.

## Configuration

The role ARN is part of the Bedrock provider settings and is set through
the AI provider API. It is optional: a provider with no role ARN behaves
exactly as before.

## Scope and trade-offs

- This PR is backend only. The settings UI for the role ARN ships in a
follow-up.
- Configuration is not exposed through environment variables.
Environment-based provider configuration is being phased out in favor of
database-managed providers, so the role ARN is intentionally database
and API only.

Follow-up PR: https://github.com/coder/coder/pull/26578

cli/aibridged.go

@@ -175,7 +175,7 @@ func BuildProviders(ctx context.Context, db database.Store, cfg codersdk.AIBridg
 		if row.Enabled {
 			enabledCount++
 		}
-		prov, err := buildAIProviderFromRow(row, keysByProvider[row.ID], cfg, metrics)
+		prov, err := buildAIProviderFromRow(ctx, row, keysByProvider[row.ID], cfg, metrics)
 		if err != nil {
 			outcome.Status = aibridged.ProviderStatusError
 			outcome.Err = err
@@ -210,6 +210,7 @@ func BuildProviders(ctx context.Context, db database.Store, cfg codersdk.AIBridg
 // Disabled: true; settings decode, key loading, and credential checks
 // are skipped because the provider will never call upstream.
 func buildAIProviderFromRow(
+	ctx context.Context,
 	row database.AIProvider,
 	keys []database.AIProviderKey,
 	cfg codersdk.AIBridgeConfig,
@@ -284,14 +285,14 @@ func buildAIProviderFromRow(
 				return nil, xerrors.Errorf("anthropic key pool: %w", err)
 			}
 		}
-		return aibridge.NewAnthropicProvider(aibridge.AnthropicConfig{
+		return aibridge.NewAnthropicProvider(ctx, aibridge.AnthropicConfig{
 			Name:             row.Name,
 			BaseURL:          row.BaseUrl,
 			KeyPool:          pool,
 			APIDumpDir:       dumpDir,
 			CircuitBreaker:   cbCfg,
 			SendActorHeaders: sendActorHeaders,
-		}, bedrock), nil
+		}, bedrock)
 
 	case database.AIProviderTypeCopilot:
 		// Copilot is always BYOK; the per-user token is supplied on each
@@ -348,6 +349,7 @@ func bedrockConfigFromRow(row database.AIProvider, settings codersdk.AIProviderS
 		AccessKeySecret: accessKeySecret,
 		Model:           bedrockSettings.Model,
 		SmallFastModel:  bedrockSettings.SmallFastModel,
+		RoleARN:         bedrockSettings.RoleARN,
 	}
 }
 

cli/aibridged_internal_test.go

@@ -267,6 +267,7 @@ func TestBuildProviders(t *testing.T) {
 			Name:    "anthropic-bedrock",
 			BaseUrl: "https://bedrock-runtime.us-west-2.amazonaws.com/",
 		}
+		roleARN := "arn:aws:iam::123456789012:role/BedrockRole"
 		settings := codersdk.AIProviderSettings{
 			Bedrock: &codersdk.AIProviderBedrockSettings{
 				Region:          "us-west-2",
@@ -274,6 +275,7 @@ func TestBuildProviders(t *testing.T) {
 				AccessKeySecret: &secret,
 				Model:           model,
 				SmallFastModel:  smallModel,
+				RoleARN:         roleARN,
 			},
 		}
 		got := bedrockConfigFromRow(row, settings)
@@ -284,6 +286,7 @@ func TestBuildProviders(t *testing.T) {
 		assert.Equal(t, secret, got.AccessKeySecret)
 		assert.Equal(t, model, got.Model)
 		assert.Equal(t, smallModel, got.SmallFastModel)
+		assert.Equal(t, roleARN, got.RoleARN)
 	})
 
 	t.Run("BedrockSettingsEmpty", func(t *testing.T) {

cli/server_aibridge_internal_test.go

@@ -741,7 +741,7 @@ func TestBuildAIProviderFromRowSetsAPIDumpDir(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			provider, err := buildAIProviderFromRow(tt.row, nil, codersdk.AIBridgeConfig{
+			provider, err := buildAIProviderFromRow(t.Context(), tt.row, nil, codersdk.AIBridgeConfig{
 				AllowBYOK:  serpent.Bool(true),
 				APIDumpDir: serpent.String(dumpDir),
 			}, nil)
@@ -755,7 +755,7 @@ func TestBuildAIProviderFromRowSetsAPIDumpDir(t *testing.T) {
 func TestBuildAIProviderFromRowBedrockWithoutSettings(t *testing.T) {
 	t.Parallel()
 
-	_, err := buildAIProviderFromRow(database.AIProvider{
+	_, err := buildAIProviderFromRow(t.Context(), database.AIProvider{
 		Enabled: true,
 		Type:    database.AIProviderTypeBedrock,
 		Name:    "bedrock-no-settings",