Bug: UI incorrectly allows attempt to rename formula series labels when there are multiple series

[benHPostHog]

Bug Description

Description:
When working with formula series in insights, the UI incorrectly allows users to click on the series label to attempt renaming, even though formula series renaming is not currently supported (see related feature request #22242).

Steps to reproduce:

1. Create an insight
2. Add multiple formula series
3. Click on the series label
   → The UI allows clicking and attempts to enter rename mode, creating a misleading experience

Expected behavior:

• The series label should not be clickable for formula series
• The UI should clearly indicate that formula series cannot currently be renamed

Actual behavior:

• The series label is clickable
• The UI allows attempting to rename, even though the functionality isn't supported

Related issues:

• Feature request for formula series renaming: Rename the Formula insights  #22242

Additional context:
This creates a confusing user experience since the UI suggests renaming is possible when it's not currently supported.

Additional context

From: https://posthoghelp.zendesk.com/agent/tickets/25305

Debug info

Kind: bug

Target area: web_analytics

Report event: http://go/ticketByUUID/c235a13d-d5ec-45ea-869d-dd577111cb95

Session: https://us.posthog.com/project/sTMFPsFhdP1Ssg/replay/0194fff5-f47b-7d5e-a660-cd61e8314540?t=2328

Exceptions: https://us.posthog.com/project/2/error_tracking?filterGroup=%7B%22type%22%3A%22AND%22%2C%22values%22%3A%5B%7B%22type%22%3A%22AND%22%2C%22values%22%3A%5B%7B%22key%22%3A%22%24session_id%22%2C%22value%22%3A%5B%220194fff5-f47b-7d5e-a660-cd61e8314540%22%5D%2C%22operator%22%3A%22exact%22%2C%22type%22%3A%22event%22%7D%5D%7D%5D%7D

Location: https://eu.posthog.com/project/45949

Sentry: http://go/sentryEU/45949

Persons-on-events mode for project: person_id_override_properties_on_events