Add deterministic Python cross-SDK interop CI path

[MisterVVP]

Motivation

• Close the tracked parity gap for a deterministic C++ ↔ Python interop validation path so cross-SDK parity signals are reproducible in CI.
• Provide an explicit, pinned CI job that consumes the public a2aproject/a2a-python repository and verifies gRPC transport interoperability tests.

Description

• Add scripts/run_python_cross_sdk_interop.sh which clones a pinned a2a-python ref, installs it into a virtualenv, builds the C++ SDK, and runs gRPC transport integration tests.
• Add a new CI job python-cross-sdk-interop to .github/workflows/ci.yml that runs the script with A2A_PYTHON_REF: v0.3.0.
• Update docs/parity-a2a.md to record the improved parity state for gRPC, mark GetExtendedAgentCard as an intentional non-goal for A2A v1 (documented), and add a Parity evidence section linking the new CI signal.

Testing

• Ran formatting check with clang-format --dry-run --Werror across tracked C/C++ files and it passed.
• Configured and built the project with CMake/Ninja (cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=RelWithDebInfo / cmake --build build --parallel) and the build completed successfully.
• Executed the test suite with ctest as part of ./scripts/verify_changes.sh, and all tests passed (127/127 tests passed).
• Ran clang-tidy via the repo script; it completed and produced warnings (no failing errors reported by the verification run).

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Codex Task