fix: guard total_cached_tokens against None in Gemini metrics

[sumleo]

What

GeminiInteractions._get_metrics assigns cache_read_tokens without a null guard:

# src/ii_agent/agents/models/google/interactions.py:981
metrics.cache_read_tokens = response_usage.total_cached_tokens

total_cached_tokens is Optional[int] and comes back as None on a cache miss, so cache_read_tokens ends up None. That value is later summed in Metrics.__add__, which raises TypeError: unsupported operand type(s) for +: 'int' and 'NoneType' when metrics are accumulated across turns.

Fix

Default to 0, matching the sibling assignments right above it (lines 976-978) and the equivalent handling in gemini.py:1439:

metrics.cache_read_tokens = response_usage.total_cached_tokens or 0

Test

Added test_cache_read_tokens_none in TestGeminiInteractionsGetMetrics, mirroring the existing test_cache_read_tokens and asserting that a None cached-token count maps to 0.

Notes

The test module currently carries a module-level pytest.skip(...) left over from an earlier refactor, so the suite is skipped in CI as-is. I confirmed the new case passes locally with the skip temporarily lifted (the whole TestGeminiInteractionsGetMetrics class is green). I left the skip untouched to keep this change scoped to the bug fix.

[sumleo]

Hi @baquy96, gentle nudge on this when you have a moment. It's a small, self-contained prompt-caching fix, and I'm happy to rebase or tweak anything if that would make review easier. Thanks for the project and your time!