Vendor google/api protos, update proto codegen, and begin RPC naming alignment

[MisterVVP]

Motivation

• Upstream a2a.proto imports google/api/*.proto symbols which were missing and caused protoc/build failures; vendoring minimal google/api protos fixes the immediate import errors so codegen can run locally.
• The project must generate C++ sources for those google API protos so generated a2a.pb.h/.cc includes resolve during build.
• The upstream proto has renamed an RPC around push-notification config; the SDK should begin tracking that naming change to reduce churn when fully aligning to the new schema.

Description

• Vendored minimal google/api proto definitions into third_party/googleapis/google/api/ (annotations.proto, client.proto, field_behavior.proto, http.proto) so import "google/api/..." resolves locally.
• Updated proto/CMakeLists.txt to add --proto_path=${A2A_GOOGLEAPIS_PROTO_ROOT}, include the vendored google/api files in the protoc invocation, and generate/compile their .pb.cc/.pb.h alongside a2a.proto outputs into the a2a_proto_generated library.
• Started API surface alignment for the push-notification RPC by renaming SDK transport/client/server symbols and tests from SetTaskPushNotificationConfig to CreateTaskPushNotificationConfig (representative files changed: include/a2a/*, src/*, examples/*, tests/*).
• Committed the above changes and recorded the work to continue a broader compatibility sweep (there are additional schema deltas in a2a.proto that still require code updates).

Testing

• Ran CMake configure with cmake -S . -B build -DCMAKE_BUILD_TYPE=RelWithDebInfo -DCMAKE_EXPORT_COMPILE_COMMANDS=ON and configuration succeeded.
• Ran clang-format --dry-run --Werror on tracked C/C++ files and it passed for the touched files.
• Ran ./scripts/verify_changes.sh; protoc codegen and a2a_proto_codegen completed and C++ proto generation succeeded, but ./scripts/verify_changes.sh ultimately failed because of broader upstream proto/schema incompatibilities (missing/renamed fields and enums in the new a2a.proto) that require a follow-up compatibility sweep; build progressed much further than before (the original import errors are resolved) but the full test/lint pipeline did not finish green.

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Codex Task