fix: close posthog clients and only create clients if telemtry is ena…

[hrisp]

Description

Every call to memory/telemetry.py:capture_event creates a new PostHog client, which internally creates a new thread to handle an internal message queue. This can lead to massive thread counts from idle PostHog clients.

The environment flag MEM0_TELEMETRY=False does not prevent this as it just pauses the PostHog client's internal queue processing.

This PR:

• adds an explicit call to AnonymousTelemetry.close shutdown the PostHog client
• honors the MEM0_TELEMETRY flag more conistently by not even creating a PostHog client.

See #3376

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

Maintainer Checklist

• [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
• [ ] Made sure Checks passed

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