fix(redact): AgentMail prefix rule matches any opaque key body, not only hex

Review finding: the salvaged rule assumed a lowercase-hex grammar that
AgentMail's docs do not establish (only the `am_` / `am_org_` prefix is
documented), so a non-hex key would have gone unmasked. Discriminate on
what actually separates keys from identifiers: an alphanumeric body with
no `_`/`-` and a 20-char floor. `am_example_identifier_123` still passes.

agent/redact.py

@@ -159,7 +159,10 @@ _PREFIX_PATTERNS = [
     r"pypi-[A-Za-z0-9_-]{10,}",         # PyPI API token
     r"dop_v1_[A-Za-z0-9]{10,}",         # DigitalOcean PAT
     r"doo_v1_[A-Za-z0-9]{10,}",         # DigitalOcean OAuth
-    r"am_[a-f0-9]{32,}",              # AgentMail API key (hex format, 32+ chars)
+    # AgentMail API key: ``am_`` / ``am_org_`` + an opaque alphanumeric body. The body has no ``_``/``-``,
+    # which is what separates it from ``am_example_identifier_123`` (#10983); public docs pin only the
+    # prefix, so the charset stays broad and the length floor does the discriminating.
+    r"am_(?:org_)?[A-Za-z0-9]{20,}",
     r"sk_[A-Za-z0-9_]{10,}",            # ElevenLabs TTS key (sk_ underscore, not sk- dash)
     r"tvly-[A-Za-z0-9]{10,}",           # Tavily search API key
     r"exa_[A-Za-z0-9]{10,}",            # Exa search API key

tests/agent/test_redact.py

@@ -63,8 +63,8 @@ class TestKnownPrefixes:
         """``am_`` is a common identifier prefix; only the documented hex key body is a secret (#10983)."""
         for benign in ["schema.am_example_identifier_123", "path/to/am_monthly_report.sql"]:
             assert redact_sensitive_text(benign) == benign
-        key = "am_" + "0123456789abcdef" * 2
-        assert "0123456789abcdef" not in redact_sensitive_text(f"AGENTMAIL_API_KEY is {key}")
+        for key in ("am_" + "0123456789abcdef" * 2, "am_" + "Ab9" * 8, "am_org_" + "Zq7k" * 6):
+            assert key[-12:] not in redact_sensitive_text(f"leaked {key} in output"), key
 
     def test_slack_token(self):
         token = "xoxb-" + "0" * 12 + "-" + "a" * 14