fix(common-server): add listener bind to prevent blocking on multiport setup

[stefan-ctrl]

Goal

Fix the Timeout bug as seen in main: https://github.com/telekom/controlplane/actions/runs/33595609763/job/100138458027#step:6:58

Description

Issue

pkg/server tests could hang for 10 minutes:

• MultiServer.Run bound each listener lazily inside its own goroutine (app.Listen(addr)), while a separate goroutine shuts down all apps exactly once when ctx is cancelled.
• If one listener's port was already in use, its bind failed instantly and cancelled ctx.
• If that happened before a sibling goroutine even got scheduled, Shutdown() was a no-op on it (fiber only supports shutdown once serving has started).
• The sibling then later bound and blocked in Accept() forever.

Root Cause

According to AI:
This race has existed since MultiServer was introduced in #507 and just never lost the coin flip in CI Verified across ~30 recent runs — all green, ~5s each.

• Original #507 run (2026-07-30)
• #587 fiber bump run (2026-08-24)
• #614 run (2026-08-31)
• 2026-08-20 run

Personal Note:
Seems kinda unlikely that nothing has changed and a "coin flip" was always in our favor, since it now fails constantly.
However, I would not consider it worth it to explore potential root causes even further, and instead fix this issue and move on.

[Copilot]

🟡 Changes recommended

The new preflight check is skipped for TLS listeners even though production uses TLS MultiServer and serve.ServeTLS still binds inside the goroutine, so the same hang scenario can still occur.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR aims to prevent common-server/pkg/server multiport server tests (and potentially servers) from hanging when one listener fails to bind by adding a synchronous preflight bind-check before starting any serve goroutines.

Changes:

• Add a preflight net.Listen/Close loop for plain-HTTP listeners to fail fast on bind errors before goroutines are spawned.
• Introduce net import and return a contextual error (listener "<addr>": ...) on preflight bind failure.

File summaries

File	Description
common-server/pkg/server/multiport.go	Adds a preflight bind-check intended to prevent a shutdown race/hang when one of multiple listeners cannot bind.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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[stefan-ctrl]

https://github.com/telekom/controlplane/actions/runs/33604567375

[ron96g]

Really minor bug that probably cannot really happen in prod, unless we misconfigure ports. Just one comment

[ron96g]

some minor things. Mainly I would use the net.Listener over the addr in most cases

[ron96g]

LGTM