fix: propagate LLM timeout to AI Core client

Address review feedback:

MAJOR:
- AICoreClient now defaults to 5min timeout (matching Client)
- Add AICoreClient.WithTimeout() for explicit timeout control
- Client.WithAICore() inherits parent client's timeout
- Client.WithTimeout() propagates to aicore client if set

MINOR:
- Extract AICoreOpenAIAPIVersion constant for the hardcoded api-version
- Tighten IsAnthropicModel to only match 'anthropic--' prefix
  (SAP AI Core always uses this prefix for Anthropic models)

NIT:
- Use fmt.Sprintf for token generation in tests (robust for >9 calls)
- Add error checking in TestAICoreClient_TokenExpiry
- Add tests for WithTimeout propagation

llm/client.go

@@ -52,8 +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
 }
 
@@ -71,9 +75,10 @@ func (c *Client) WithProvider(p Provider) *Client {
 
 // 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)
+	c.aicore = NewAICoreClient(cfg).WithTimeout(c.http.Timeout)
 	return c
 }
 
@@ -201,11 +206,11 @@ 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"`
+	Model       string         `json:"model"`
+	MaxTokens   int            `json:"max_tokens"`
+	System      string         `json:"system,omitempty"`
+	Messages    []anthropicMsg `json:"messages"`
+	Temperature float64        `json:"temperature,omitempty"`
 }
 
 type anthropicMsg struct {