fix(skills): prevent empty responses after load_skill

[lwangverizon]

fix(skills): prevent empty responses after load_skill

Fixes #6225

Problem

When an agent uses the skill tools, ADK appends a skill system instruction to the
prompt (_build_skill_system_instruction in
src/google/adk/tools/skill_toolset.py). The instruction does not tell the model
what to do after a load_skill call returns. With Gemini models we observed a
significant rate of empty model responses right after a load_skill call — the
model loads the skill and ends its turn with no visible output.

We saw this as a spike in empty responses in a Verizon production environment running
ADK + Gemini, most acutely for tool-heavy skills whose next correct action after
loading is to call more tools rather than reply.

Change

_build_skill_system_instruction only — covers both DEFAULT_SKILL_SYSTEM_INSTRUCTION
and the per-toolset prefixed variant.

Add rule 7 (continue-after-load guard), appended after the existing rule 6:

7. Loading a skill only retrieves its instructions; it does NOT complete your turn.
   After a load_skill call returns, continue in the SAME turn: call whatever tools
   the skill's steps require (search, data retrieval, render), then write your reply.
   Never end your turn with an empty response right after loading a skill.

No other rule is modified — rules 1–6 are left exactly as-is. The new rule uses the
{prefix} substitution like the other rules ({p}load_skill), so the prefixed system
instruction carries it too.

Result

This is the exact change we validated in production: appending rule 7 (rules 1–6
unchanged) dropped the empty-response rate for Gemini after skill loading
substantially.

Tests

Adds focused tests in tests/unittests/tools/test_skill_toolset.py:

• test_system_instruction_marks_load_skill_as_non_terminal — rule 7 present in the
  default instruction.
• test_prefixed_system_instruction_includes_continue_after_load_rule — prefixed
  variant carries rule 7 with the prefix.

Existing tests reference DEFAULT_SKILL_SYSTEM_INSTRUCTION by constant (not hardcoded
text), so they are unaffected.

Backwards compatibility

Prompt-text-only, strictly additive change. No public API, signature, or existing-rule
change.