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Zoltan Papp b6cd8944b1 [client] Fix nil context panic in iOS dynamic route resolver (#6848 )

## Describe your changes

getIPsFromResolver passed a nil context to ExchangeWithFallback, which
net.Dialer.DialContext rejects with panic("nil context"). On iOS this
crashed the whole network extension (SIGABRT) ~2 seconds after connect
whenever the network map contained a domain-based (dynamic) route, as
the resolver goroutine panicked on its first DNS query.

Passing nil used to be a documented input of ExchangeWithFallback ("If
the passed context is nil, this will use Exchange instead of
ExchangeContext") since #3632. 9ed2e2a5b (#5971) removed the nil-context
branch, but this iOS-only caller was not updated — it never fails CI
since route_ios.go only builds with GOOS=ios. Broken since v0.71.1.

Pass a context bounded by the existing dialTimeout instead, matching the
dnsinterceptor pattern (context.Background() + timeout).

Captured panic (netbird.err):
  panic: nil context
  net.(*Dialer).DialContext -> miekg/dns ExchangeContext ->
nbdns.ExchangeWithFallback(nil, ...) ->
dynamic.(*Route).getIPsFromResolver
  route_ios.go:35


## Issue ticket number and link

## Stack

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### Checklist
- [x] Is it a bug fix
- [ ] Is a typo/documentation fix
- [ ] Is a feature enhancement
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- [ ] This change does **not** modify the public API, gRPC protocols,
functionality behavior, CLI / service flags, or introduce a new feature
— **OR** I have discussed it with the NetBird team beforehand (link the
issue / Slack thread in the description). See
[CONTRIBUTING.md](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTING.md#discuss-changes-with-the-netbird-team-first).

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## Documentation
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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Fixed iOS dynamic DNS resolution by ensuring DNS queries use a proper
resolver context instead of a nil one.
* Improved DNS resolution reliability by propagating
cancellation/timeouts through all domain IP lookups, including fallback
system resolver queries.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->