[DBMON-6784] Fix Clickhouse database_hostname usage

[eric-weaver]

What does this PR do?

Adds a dedicated database_hostname property to the ClickHouse check and uses it for the database_hostname tag and the database-instance metadata, instead of reusing reported_hostname.

The new property always resolves the underlying database host via resolve_db_host(server), independent of the reported_hostname config override, and is cached. This mirrors the database_hostname convention already implemented in the Postgres, MySQL, and SQL Server integrations.

Changes:

• Add cached database_hostname property (initialized in __init__).
• Source the database_hostname core tag (_add_core_tags) and the database_hostname metadata field (_send_database_instance_metadata) from self.database_hostname rather than self.reported_hostname.
• Update unit tests to assert against check.database_hostname.

Motivation

ClickHouse conflated database_hostname with reported_hostname. When a user set the reported_hostname config override, the database_hostname tag and metadata incorrectly became that override value instead of the true resolved host. database_hostname is meant to be a stable identifier of the underlying DB host used to correlate DBM data, so this broke correlation and diverged from the behavior of the other DBM integrations.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[datadog-prod-us1-3]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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[dd-octo-sts]

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