fix: pin @google/genai and google-auth-library to avoid gaxios v7 stream corruption

google-auth-library v10+ depends on gaxios v7.1.3 which has a bug:
its _request() method reads stream error bodies using Array.toString(),
which joins Buffer chunks with commas — corrupting JSON error responses
at TCP chunk boundaries. This causes TerminalQuotaError to be
misclassified due to corrupted @type fields in error details.

This fix reverts the dependency bumps from #18880:
- @google/genai: 1.41.0 → 1.30.0 (packages/core, packages/cli, packages/a2a-server)
- google-auth-library: ^10.5.0 → ^9.11.0 (packages/core)

google-auth-library v9.x uses gaxios v6.x which correctly concatenates
chunks via string addition (response += chunk).

DO NOT upgrade these packages until the gaxios v7 bug is fixed upstream.
See: #21884

Author: gsquared94