Fix Gemma tool declaration echoes

[h30s]

#Fixes #693

Changes:

• Blocks Gemma tool declaration echo paths when tools are present.
• Preserves optional tool behavior unless force_tools is true.
• Keeps full grammar forcing for force_tools=true.
• Tightens LLM tests so empty function_calls no longer pass.

Validation:

• git diff --check
• Local branch-logic simulation passed.
• Could not run cactus test --suite llm locally because this machine/toolchain is x86 while cactus-kernels requires ARM NEON flags/headers.

Reviewer request:
Please run cactus test --suite llm on an ARM/Gemma4 setup and confirm tool cases produce non-empty function_calls and no <|tool>declaration: echo."