fix(agents): default-deny args keys in from_config() YAML loads

[vexlorn]

Summary

• from_config(config_path, *, trusted=False) rejects YAML configs containing args keys by default
• _load_config_from_path accepts a matching trusted parameter; gate logic is if not trusted or _ENFORCE_DENYLIST
• External-facing call sites (cli_deploy.py Agent Engine deployment template, cli/utils/agent_loader.py) inherit the secure default with no caller-side change
• Operator-controlled trusted loaders pass trusted=True explicitly
• Legacy _set_enforce_denylist(True) preserved as a force-on override; _set_enforce_denylist(False) is a no-op since the default is now block

This is one of the three fixes proposed in #5532. The other two (default-on of the existing global flag, UntrustedCodeConfig type split) are larger or breaking; this one is the smallest non-breaking change that closes the sink at the boundary.

Threat model

config_agent_utils.from_config() and its private callee _load_config_from_path() reach resolve_code_reference(), which imports a Python callable named in YAML and invokes it with attacker-supplied positional and keyword arguments when the YAML contains an args key. v1.31.1 added _check_yaml_for_blocked_keys but gated it behind an opt-in flag that is not set by the call sites listed below.

Reachable from:

• src/google/adk/cli/cli_deploy.py:113 — Agent Engine deployment template: root_agent = config_agent_utils.from_config(config_path)
• src/google/adk/cli/utils/agent_loader.py:173 — _load_from_yaml_config() for adk run, adk web, adk deploy

After this PR, both call sites use the default trusted=False and the denylist runs.

Test plan

• test_agent_config_litellm_model_with_custom_args — updated to pass trusted=True (operator-authored model_code with class-constructor args; legitimate)
• test_agent_config_legacy_model_mapping_still_supported — updated to pass trusted=True (same legitimate pattern, legacy field mapping)
• test_load_config_from_path_blocks_args_when_enforced — unchanged; legacy global flag still triggers block
• test_from_config_blocks_args_by_default — new; default from_config() rejects args
• test_from_config_allows_args_when_trusted — new; trusted=True accepts the same YAML
• test_from_config_default_blocks_os_system_in_output_schema — new; concrete RCE PoC blocked, marker file not created

All 35 tests in tests/unittests/agents/test_agent_config.py pass locally (pytest -v).

Out of scope

• resolve_agent_reference() (sub-agent loading) does not currently propagate trusted. If the parent is trusted=True, sub-agents loaded via config_path revert to default trusted=False. Maintainer to decide propagation policy.
• UntrustedCodeConfig type split (issue option 3) — separate larger PR.
• Removal of legacy _set_enforce_denylist API — separate cleanup.

Disclosure

Reported via g.co/vulnz on 2026-04-18 (Issue Tracker #503880658). Closed Won't Fix (Intended Behavior) on 2026-04-28. Filing per maintainer guidance to pursue fix via public channels.

Refs #5532.