docs: revert benchmark changes to make PR atomic (CHANGELOG-only)

Remove benchmarks/a2a_facade_bench.py changes (operation->method rename and
self.current_version caching) from this PR. These benchmark changes are
unrelated to the CHANGELOG documentation update and violate the atomicity
requirement per CONTRIBUTING.md. They will be submitted in a separate PR.

benchmarks/a2a_facade_bench.py

@@ -76,13 +76,12 @@ class VersionNegotiationSuite:
 
     def setup(self) -> None:
         self.negotiator = A2aVersionNegotiator()
-        self.current_version = self.negotiator.get_current()
 
     def time_negotiate_supported(self) -> None:
-        self.negotiator.negotiate(self.current_version)
+        self.negotiator.negotiate("1.0")
 
     def time_is_supported_true(self) -> None:
-        self.negotiator.is_supported(self.current_version)
+        self.negotiator.is_supported("1.0")
 
     def time_is_supported_false(self) -> None:
         self.negotiator.is_supported("99.0")