A2A tool responses coalesced to prevent 400 errors

What's Changed

• chore/release: bump version to 0.52.0-nightly.20260715.gfa975395b by @gemini-cli-robot in https://github.com/google-gemini/gemini-cli/pull/28402
• fix(core,a2a): group cancelled tool responses and coalesce consecutive roles to prevent 400 Bad Request by @luisfelipe-alt in https://github.com/google-gemini/gemini-cli/pull/28407

Full Changelog: https://github.com/google-gemini/gemini-cli/compare/v0.52.0-nightly.20260715.gfa975395b...v0.52.0-nightly.20260716.g3ff5ba20f