[codex] fix(gemini): record tool use prompt token telemetry

[gold-silver-copper]

Summary

Follow-up to #1790.

• Records normalized Usage.tool_use_prompt_tokens as gen_ai.usage.tool_use_prompt_tokens in the shared telemetry span recorder.
• Declares the field on Gemini completion/streaming spans so the shared recorder can emit it for Gemini usage metadata.
• Records the aggregated tool-use prompt token count on agent prompt and streaming spans.
• Adds a focused telemetry unit test that verifies the shared recorder emits the new span field.

Why

#1790 exposed Gemini toolUsePromptTokenCount through normalized usage, but telemetry spans still only recorded input, output, cache, and reasoning token counts. That meant span-based analytics consumers could miss the newly normalized token count.

Validation

• cargo fmt --check
• cargo test -p rig-core telemetry::tests::record_token_usage_records_tool_use_prompt_tokens
• cargo test -p rig-core gemini
• cargo clippy -p rig-core --all-targets --all-features
• cargo test -p rig-core
• git diff --check