feat: native SAP AI Core support

Add native SAP AI Core provider that handles OAuth token management and
deployment discovery automatically. This eliminates the need for the
external LLM proxy when running in SAP environments.

Changes:
- Add AICoreClient with OAuth token caching and deployment URL discovery
- Support both Anthropic and OpenAI models via AI Core deployments
- Update CI to use native AI Core provider
- Update action inputs to accept AI Core credentials
- Update README with AI Core configuration examples

Model names must match AI Core deployment names (e.g. anthropic--claude-4.6-sonnet, gpt-5).

llm/client.go

@@ -1,6 +1,6 @@
 // Package llm provides clients for LLM chat completion APIs.
 //
-// Supports OpenAI-compatible (default) and Anthropic Messages API providers.
+// Supports OpenAI-compatible (default), Anthropic Messages API, and SAP AI Core providers.
 package llm
 
 import (
@@ -22,6 +22,8 @@ const (
 	ProviderOpenAI Provider = "openai"
 	// ProviderAnthropic uses the Anthropic Messages API endpoint.
 	ProviderAnthropic Provider = "anthropic"
+	// ProviderAICore uses SAP AI Core with OAuth authentication.
+	ProviderAICore Provider = "aicore"
 )
 
 // Client calls an LLM chat completion API.
@@ -35,6 +37,7 @@ type Client struct {
 	temperature float64
 	provider    Provider
 	http        *http.Client
+	aicore      *AICoreClient // Only set when provider is aicore
 }
 
 // NewClient creates a new LLM client. Default provider is OpenAI-compatible.
@@ -49,8 +52,12 @@ func NewClient(baseURL, apiKey, model string) *Client {
 }
 
 // WithTimeout sets the HTTP request timeout for LLM calls (default 5 minutes).
+// When using AI Core, this also sets the timeout on the AI Core client.
 func (c *Client) WithTimeout(d time.Duration) *Client {
 	c.http.Timeout = d
+	if c.aicore != nil {
+		c.aicore.WithTimeout(d)
+	}
 	return c
 }
 
@@ -60,12 +67,21 @@ func (c *Client) WithTemperature(t float64) *Client {
 	return c
 }
 
-// WithProvider sets the API provider format (openai or anthropic).
+// WithProvider sets the API provider format (openai, anthropic, or aicore).
 func (c *Client) WithProvider(p Provider) *Client {
 	c.provider = p
 	return c
 }
 
+// WithAICore configures the client to use SAP AI Core for authentication.
+// This sets the provider to aicore automatically.
+// The AI Core client inherits the current HTTP timeout from this client.
+func (c *Client) WithAICore(cfg AICoreConfig) *Client {
+	c.provider = ProviderAICore
+	c.aicore = NewAICoreClient(cfg).WithTimeout(c.http.Timeout)
+	return c
+}
+
 // Message represents a chat message.
 type Message struct {
 	Role    string `json:"role"`
@@ -82,6 +98,8 @@ func (c *Client) Complete(ctx context.Context, messages []Message) (string, erro
 		switch c.provider {
 		case ProviderAnthropic:
 			result, err = c.completeAnthropic(ctx, messages)
+		case ProviderAICore:
+			result, err = c.completeAICore(ctx, messages)
 		default:
 			result, err = c.completeOpenAI(ctx, messages)
 		}
@@ -106,6 +124,18 @@ func (c *Client) Complete(ctx context.Context, messages []Message) (string, erro
 	return "", err
 }
 
+// completeAICore routes to AI Core using the appropriate endpoint based on model type.
+func (c *Client) completeAICore(ctx context.Context, messages []Message) (string, error) {
+	if c.aicore == nil {
+		return "", fmt.Errorf("AI Core client not configured")
+	}
+
+	if IsAnthropicModel(c.model) {
+		return c.aicore.CompleteAnthropic(ctx, c.model, messages, 8192, c.temperature)
+	}
+	return c.aicore.CompleteOpenAI(ctx, c.model, messages, c.temperature)
+}
+
 // isRetryableError returns true for transient errors worth retrying.
 func isRetryableError(err error) bool {
 	if err == nil {
@@ -176,11 +206,12 @@ func (c *Client) completeOpenAI(ctx context.Context, messages []Message) (string
 // --- Anthropic Messages API implementation ---
 
 type anthropicRequest struct {
-	Model       string            `json:"model"`
-	MaxTokens   int               `json:"max_tokens"`
-	System      string            `json:"system,omitempty"`
-	Messages    []anthropicMsg    `json:"messages"`
-	Temperature float64           `json:"temperature,omitempty"`
+	AnthropicVersion string         `json:"anthropic_version,omitempty"`
+	Model       string         `json:"model,omitempty"`
+	MaxTokens   int            `json:"max_tokens"`
+	System      string         `json:"system,omitempty"`
+	Messages    []anthropicMsg `json:"messages"`
+	Temperature float64        `json:"temperature,omitempty"`
 }
 
 type anthropicMsg struct {