Share strict database server port parsing

Is your feature request related to a problem? Please describe.

R2DBC, ClickHouse, and Redis each contain nearly identical parsing for configured database server ports. Keeping separate copies makes their validation rules easy to drift.

Describe the solution you'd like

Add a shared DbServerEndpointUtil.parsePort(String) helper that accepts only ASCII decimal ports in the range 1..65535. Reuse it from R2dbcServerTarget, ClickHouseClientV2Singletons, and RedisServerTarget.

Describe alternatives you've considered

Leave the parsers local, or migrate every existing port parser at once. The older JDBC, HBase, Couchbase, and OpenSearch helpers have different contracts for trimming, range checks, logging, and sentinel values, so changing those should be considered separately.

Additional context

Follow-up to the review discussion in #19981: https://github.com/open-telemetry/opentelemetry-java-instrumentation/pull/19981#discussion_r3976844555

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