fix(transport/rust): migrate v0.56 debug-aware patch from test-plans #797

[acul71]

Summary

• Migrate the effective patch from libp2p/test-plans#797 into libp2p/unified-testing.
• Update transport/images/rust/v0.56/transport-fix.patch to include debug-flag-aware behavior and aligned env/config naming used by the transport harness.
• Keep the migration scoped to the single patch file (exclude merge-only history from the source PR).

Context

• Original PR: Fix/enable debug in rust v0.56 test-plans#797
• Migration guidance: Migration Required: Moving Open PRs from `test-plans` → `unified-testing` #21

Test plan

• Run transport tests with rust-v0.56 as listener and dialer.
• Validate debug output behavior with debug enabled/disabled.
• Confirm no regression in non-debug execution path.

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