fix(vmm): reject conflicting host port mappings

[kvinwang]

Problem

Two VM/app definitions could reserve the same host address, port, and protocol because validation considered each mapping in isolation. Both configurations were accepted and the second launch later failed to bind, or traffic reached whichever listener won.

Root cause and fix

Normalize host-port tuples during validation and reject duplicates before persisting or launching the VM.

Implementation

The branch records the following focused implementation work:

• fix(vmm): reject conflicting host port mappings

Changed paths:

• dstack/vmm/src/main_service.rs

Scope

This PR addresses one logical vmm finding. It intentionally excludes the acceptance-test infrastructure from #841 and unrelated product fixes from #840.

Dependency and merge order

This PR is based directly on master and does not require another split product PR to merge first.

Verification

• git diff --check origin/master..origin/codex/fix-vmm-host-port-conflicts: passed.
• The declared base was verified as an ancestor of the PR head.
• Focused compile/check verification was run for the changed component where applicable; non-Rust packaging or configuration changes were reviewed against their exact branch delta.

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