fix(node): Use snake_case for Fastify's request-handler op.

[onurtemizkan]

Following up: #18617

The span ops should be using snake_case as per our spec: https://develop.sentry.dev/sdk/telemetry/traces/span-operations/

Closes #18730 (added automatically)

[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	9,464	-	10,769	-12%
GET With Sentry	1,753	19%	1,658	+6%
GET With Sentry (error only)	6,142	65%	7,175	-14%
POST Baseline	1,203	-	1,061	+13%
POST With Sentry	602	50%	491	+23%
POST With Sentry (error only)	1,073	89%	961	+12%
MYSQL Baseline	3,353	-	3,653	-8%
MYSQL With Sentry	466	14%	403	+16%
MYSQL With Sentry (error only)	2,730	81%	2,915	-6%

View base workflow run

[Lms24]

thanks!