fix(udp): expose UDP ports properly

[blueprismo]

What does this PR do?

Fixes UDP port exposure.

Problem:
When using ExposedPorts: []string{"8080/udp"} in testcontainers-go, the MappedPort() function always returned "0/udp" instead of the actual mapped host port (e.g., "55051/udp"). This made it impossible to connect to UDP services running in containers.

Root Cause:
The issue was in the mergePortBindings() function in lifecycle.go. When nat.ParsePortSpecs() processes exposed port specifications like 8080/udp, it creates PortBinding structs with empty HostPort fields ({HostIP: "", HostPort: ""})
Docker interprets these differently:

HostPort: "" (empty) → "Don't bind this port to any host port" → results in port 0
HostPort: "0" → "Bind this port to a random available host port" → proper allocation
Solution:
Modified mergePortBindings() to convert empty HostPort values to "0" for automatic port allocation:

Why is it important?

To support UDP connections correctly as now they aren't useful (at least with random port allocatio

Related issues

Follows: #3484

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[coderabbitai]

Summary by CodeRabbit

Bug Fixes

• Fixed UDP port binding to correctly signal Docker for automatic random port allocation when no specific port is specified.

Tests

• Added comprehensive test coverage for UDP port binding scenarios and port allocation behavior.

Walkthrough

A fix is applied to the mergePortBindings function to normalize empty HostPort values to "0", enabling Docker to allocate random ports automatically. Comprehensive tests are added to validate UDP port binding behavior and document port parsing edge-cases, with test expectations updated to reflect the normalized HostPort values.

Changes

Cohort / File(s)	Summary
UDP Port Binding Fix lifecycle.go	Post-processes exposed port bindings in mergePortBindings to convert empty HostPort values to "0" for automatic Docker port allocation, fixing UDP port-binding behavior
Test Suite for UDP Port Binding udp_port_binding_test.go	New test file with comprehensive coverage: UDP auto-allocation via MappedPort, TCP regression tests, mergePortBindings unit tests validating empty-to-"0" conversion, and documentation-style tests for nat.ParsePortSpecs behavior
Updated Test Expectations lifecycle_test.go	Updates test expectations in PreCreateModifierHook and TestMergePortBindings scenarios to use "0" instead of "" for HostPort values in port binding mappings

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

• lifecycle.go: The production fix is localized and straightforward—a simple post-processing step normalizing HostPort values. Verify the logic handles all edge-cases and doesn't inadvertently affect existing bindings.
• udp_port_binding_test.go: The new test suite is comprehensive and well-structured, but requires careful review to ensure all UDP and TCP scenarios are adequately covered and assertions are correct.
• lifecycle_test.go: Test expectation changes are data-driven updates; cross-reference with the production fix to confirm all expectations align with the new "0" default.

Poem

🐰 A rabbit hops through port allocation,
Where empty strings caused frustration.
Now "0" whispers to Docker's ear,
"Assign a port, make it clear!"
UDP flows and TCP stays true—
All ports now have work to do! 🐇

Pre-merge checks and finishing touches

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description provides a clear explanation of the problem, root cause, and solution related to UDP port binding in testcontainers-go, directly addressing the changeset.
Title check	✅ Passed	The title 'fix(udp): expose UDP ports properly' directly addresses the main change—fixing UDP port exposure in testcontainers-go by converting empty HostPort values to '0' for proper auto-allocation.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[mdelapenya]

@blueprismo i think we need to update the tests for the new condition. Can you take a look? 🙏