Emit low cardinality gen_ai agent span names

[Lms24]

Ensure gen_ai agent spans emit low-cardinality span names when span streaming is enabled.

Span name rules: https://getsentry.github.io/sentry-conventions/names/#gen_ai-agent

Fallback constant: GEN_AI_AGENT_SPAN_NAME_FALLBACK in packages/core/src/tracing/spans/spanNames.ts

Part of #22350 — remember to check traceLifecycle and keep the existing name for static mode.

[linear-code]

JS-3409

[RulaKhaled]

Blocked by agent-monitoring sign-off on the v11 low-cardinality gen_ai names