fix(core): Flatten gen_ai.request.available_tools in google-genai

[nicohrubec]

While investigating this ticket I noticed that available tools are sent as a nested instead of a flat array in google genai, which seems like a bug to me.

The format I would expect and how we do it in other integrations is:
[{tool-definition}, {tool-definition}]

What we actually send atm is:
[[{tool-definition}], [{tool-definition}]]

This PR fixes this to instead send a flat list of tool definitions.

[github-actions]

node-overhead report 🧳

Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.

Scenario	Requests/s	% of Baseline	Prev. Requests/s	Change %
GET Baseline	8,790	-	8,922	-1%
GET With Sentry	1,319	15%	1,350	-2%
GET With Sentry (error only)	6,066	69%	6,147	-1%
POST Baseline	1,176	-	1,205	-2%
POST With Sentry	495	42%	524	-6%
POST With Sentry (error only)	1,063	90%	1,072	-1%
MYSQL Baseline	3,307	-	3,368	-2%
MYSQL With Sentry	471	14%	517	-9%
MYSQL With Sentry (error only)	2,674	81%	2,772	-4%

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