fix: add WhatsApp webhook signature verification (X-Hub-Signature-256)

Closes #51

- Add HMAC-SHA256 signature verification for WhatsApp webhooks
- Prevents message spoofing attacks (CWE-345)
- Add whatsapp_app_secret config field with ZEROCLAW_WHATSAPP_APP_SECRET env override
- Add 13 comprehensive unit tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/providers/mod.rs

@@ -11,6 +11,84 @@ pub use traits::Provider;
 use compatible::{AuthStyle, OpenAiCompatibleProvider};
 use reliable::ReliableProvider;
 
+const MAX_API_ERROR_CHARS: usize = 200;
+
+fn is_secret_char(c: char) -> bool {
+    c.is_ascii_alphanumeric() || matches!(c, '-' | '_' | '.' | ':')
+}
+
+fn token_end(input: &str, from: usize) -> usize {
+    let mut end = from;
+    for (i, c) in input[from..].char_indices() {
+        if is_secret_char(c) {
+            end = from + i + c.len_utf8();
+        } else {
+            break;
+        }
+    }
+    end
+}
+
+/// Scrub known secret-like token prefixes from provider error strings.
+///
+/// Redacts tokens with prefixes like `sk-`, `xoxb-`, and `xoxp-`.
+pub fn scrub_secret_patterns(input: &str) -> String {
+    const PREFIXES: [&str; 3] = ["sk-", "xoxb-", "xoxp-"];
+
+    let mut scrubbed = input.to_string();
+
+    for prefix in PREFIXES {
+        let mut search_from = 0;
+        loop {
+            let Some(rel) = scrubbed[search_from..].find(prefix) else {
+                break;
+            };
+
+            let start = search_from + rel;
+            let content_start = start + prefix.len();
+            let end = token_end(&scrubbed, content_start);
+
+            // Bare prefixes like "sk-" should not stop future scans.
+            if end == content_start {
+                search_from = content_start;
+                continue;
+            }
+
+            scrubbed.replace_range(start..end, "[REDACTED]");
+            search_from = start + "[REDACTED]".len();
+        }
+    }
+
+    scrubbed
+}
+
+/// Sanitize API error text by scrubbing secrets and truncating length.
+pub fn sanitize_api_error(input: &str) -> String {
+    let scrubbed = scrub_secret_patterns(input);
+
+    if scrubbed.chars().count() <= MAX_API_ERROR_CHARS {
+        return scrubbed;
+    }
+
+    let mut end = MAX_API_ERROR_CHARS;
+    while end > 0 && !scrubbed.is_char_boundary(end) {
+        end -= 1;
+    }
+
+    format!("{}...", &scrubbed[..end])
+}
+
+/// Build a sanitized provider error from a failed HTTP response.
+pub async fn api_error(provider: &str, response: reqwest::Response) -> anyhow::Error {
+    let status = response.status();
+    let body = response
+        .text()
+        .await
+        .unwrap_or_else(|_| "<failed to read provider error body>".to_string());
+    let sanitized = sanitize_api_error(&body);
+    anyhow::anyhow!("{provider} API error ({status}): {sanitized}")
+}
+
 /// Factory: create the right provider from config
 #[allow(clippy::too_many_lines)]
 pub fn create_provider(name: &str, api_key: Option<&str>) -> anyhow::Result<Box<dyn Provider>> {
@@ -394,4 +472,92 @@ mod tests {
             );
         }
     }
+
+    // ── API error sanitization ───────────────────────────────
+
+    #[test]
+    fn sanitize_scrubs_sk_prefix() {
+        let input = "request failed: sk-1234567890abcdef";
+        let out = sanitize_api_error(input);
+        assert!(!out.contains("sk-1234567890abcdef"));
+        assert!(out.contains("[REDACTED]"));
+    }
+
+    #[test]
+    fn sanitize_scrubs_multiple_prefixes() {
+        let input = "keys sk-abcdef xoxb-12345 xoxp-67890";
+        let out = sanitize_api_error(input);
+        assert!(!out.contains("sk-abcdef"));
+        assert!(!out.contains("xoxb-12345"));
+        assert!(!out.contains("xoxp-67890"));
+    }
+
+    #[test]
+    fn sanitize_short_prefix_then_real_key() {
+        let input = "error with sk- prefix and key sk-1234567890";
+        let result = sanitize_api_error(input);
+        assert!(!result.contains("sk-1234567890"));
+        assert!(result.contains("[REDACTED]"));
+    }
+
+    #[test]
+    fn sanitize_sk_proj_comment_then_real_key() {
+        let input = "note: sk- then sk-proj-abc123def456";
+        let result = sanitize_api_error(input);
+        assert!(!result.contains("sk-proj-abc123def456"));
+        assert!(result.contains("[REDACTED]"));
+    }
+
+    #[test]
+    fn sanitize_keeps_bare_prefix() {
+        let input = "only prefix sk- present";
+        let result = sanitize_api_error(input);
+        assert!(result.contains("sk-"));
+    }
+
+    #[test]
+    fn sanitize_handles_json_wrapped_key() {
+        let input = r#"{"error":"invalid key sk-abc123xyz"}"#;
+        let result = sanitize_api_error(input);
+        assert!(!result.contains("sk-abc123xyz"));
+    }
+
+    #[test]
+    fn sanitize_handles_delimiter_boundaries() {
+        let input = "bad token xoxb-abc123}; next";
+        let result = sanitize_api_error(input);
+        assert!(!result.contains("xoxb-abc123"));
+        assert!(result.contains("};"));
+    }
+
+    #[test]
+    fn sanitize_truncates_long_error() {
+        let long = "a".repeat(400);
+        let result = sanitize_api_error(&long);
+        assert!(result.len() <= 203);
+        assert!(result.ends_with("..."));
+    }
+
+    #[test]
+    fn sanitize_truncates_after_scrub() {
+        let input = format!("{} sk-abcdef123456 {}", "a".repeat(190), "b".repeat(190));
+        let result = sanitize_api_error(&input);
+        assert!(!result.contains("sk-abcdef123456"));
+        assert!(result.len() <= 203);
+    }
+
+    #[test]
+    fn sanitize_preserves_unicode_boundaries() {
+        let input = format!("{} sk-abcdef123", "こんにちは".repeat(80));
+        let result = sanitize_api_error(&input);
+        assert!(std::str::from_utf8(result.as_bytes()).is_ok());
+        assert!(!result.contains("sk-abcdef123"));
+    }
+
+    #[test]
+    fn sanitize_no_secret_no_change() {
+        let input = "simple upstream timeout";
+        let result = sanitize_api_error(input);
+        assert_eq!(result, input);
+    }
 }