fix(rds): fix default port in proxy

[tiksangng]

Issue # (if applicable)

Closes #37635.

Reason for this change

To fix a synthesis crash (Cannot call allowDefaultPortFrom(): this resource has no default port) that occurs because DatabaseProxy fails to inherit the default port from its underlying DatabaseCluster or DatabaseInstance target.

Description of changes

• Exposed _defaultPort on ProxyTarget to allow the target's port to be read internally before the proxy binds to it.
• Initialized the ec2.Connections object inside DatabaseProxy with the target's defaultPort.

Describe any new or updated permissions being added

N/A

Description of how you validated changes

• Added new unit tests to ensure the default port can be inferred from the proxy targets.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[maintainer-for-aws]

Automated review

A maintainer will still review this — treat the notes below as a starting point.

This PR fixes a broken abstraction in aws-rds: a DatabaseProxy never propagated its target database's port into its ec2.Connections, so proxy.connections.allowDefaultPortFrom(...) / allowDefaultPortFromAnyIpv4(...) always threw "this resource has no default port". The fix adds an @internal ProxyTarget._defaultPort getter and seeds the proxy's Connections with the target's port (an Endpoint.Port token, matching real RDS Proxy behavior where the proxy listens on the DB engine's port). The change is small, correctly ordered so bind() still sees the proxy's security groups, and preserves prior behavior for imported targets that report no port. Two new unit tests (instance and cluster targets) and an integ update exercise the fixed path with regression-catching assertions on the resulting ingress rule. No issues found on this change.

🔴 0 blocking · 🟡 0 recommended · ⚪ 0 optional

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