Added validate_domain_tags() and check_domain_title_keywords() to enforce strict one-tag-per-domain
for @a2a, @session, @cli tags in Behave tests. The only allowed cross-domain combination is
@session + @cli. Soft warnings for missing domain tags when scenario titles contain keywords.

Full BDD coverage in features/testing/bdd_tag_enforcement.feature with 18 scenarios.

ISSUES CLOSED: #9234

@@ -59,0 +63,4 @@
+# Keywords that indicate the scenario belongs to a domain but is missing the tag.
+# Each keyword maps to the canonical domain tag it should have been tagged with.
+_DOMAIN_KEYWORDS: dict[str, str] = {
+    "a2a": "a2a",

@@ -59,0 +107,4 @@
+    if domain_count <= 1:
+        return  # Valid: single tag or none.
+
+    # At this point domain_count == 2 (can't be more -- only three domains).

@@ -59,0 +118,4 @@
+        offending: list[str] = []
+        if has_a2a:
+            offending.append("@a2a")
+        if has_session:

@@ -0,0 +132,4 @@
+
+# ---------------------------------------------------------------------------
+# Mock-domain scenario builders for edge cases
+# ---------------------------------------------------------------------------

@@ -59,0 +68,4 @@
+    "inter-agent": "a2a",
+    "client agent": "a2a",
+    "session": "session",
+    "new session": "session",

@@ -59,0 +76,4 @@
+    "cli command": "cli",
+}
+_DOMAIN_KEYWORD_RE = re.compile(
+    "|".join(rf"(?:{k})" for k in _DOMAIN_KEYWORDS),

@@ -59,0 +129,4 @@
+            "The only allowed cross-domain combination is @session + @cli. "
+            "See CONTRIBUTING.md > BDD Domain Tagging."
+        )
+