Fix handling of CLICKHOUSE_MIGRATION_URL when clickhouse.host defined

[jamesmaidment]

Fixes #240

When clickhouse.host is defined, the CLICKHOUSE_MIGRATION_URL was being constructed by directly concatenating clickhouse:// with the host value. If the host already contained a protocol (like https://), this resulted in an invalid URL.

Eg:

CLICKHOUSE_MIGRATION_URL: clickhouse://https://<redacted>.us-central1.gcp.clickhouse.cloud:9440
CLICKHOUSE_URL: https://<redacted>.us-central1.gcp.clickhouse.cloud:8443

This uses langfuse.clickhouse.hostname instead of .Values.clickhouse.host in the helper to fix handling of CLICKHOUSE_MIGRATION_URL when clickhouse.host is defined

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