feat(net): add isPortAvailable

[Xavrir]

Closes #7196.

@std/net only had getAvailablePort, so there was no quick way to ask "is this specific port free?" without writing the Deno.listen plus catch yourself. This adds that:

isPortAvailable(port: number, options?: { hostname?: string }): boolean

It returns true when the TCP port can be listened on, false on Deno.errors.AddrInUse, and rethrows any other error (including PermissionDenied for privileged ports or a missing net permission). hostname defaults to 0.0.0.0. Since net is stable, it lives in unstable_is_port_available.ts as the @std/net/unstable-is-port-available export and isn't re-exported from mod.ts.

The doc comment is upfront about the obvious race: the port can be taken between the check and your own listen, so it points back to port: 0 / getAvailablePort for cases where you own the listener.

Tests cover an available port, a port already in use, the hostname option, and the rethrow path for errors other than AddrInUse. deno task lint, deno fmt --check, and deno test -A --doc pass locally.

One open question: happy to also add getAvailablePorts(count, options?) here if that would be useful, or keep this PR to the single function.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.84%. Comparing base (0e5b1f2) to head (c996483).

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #7204    +/-   ##
========================================
  Coverage   94.83%   94.84%            
========================================
  Files         617      618     +1     
  Lines       51674    51438   -236     
  Branches     9350     9305    -45     
========================================
- Hits        49007    48785   -222     
+ Misses       2121     2112     -9     
+ Partials      546      541     -5     

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[yuhr]

Great work! Looks good to me, Thanks. I'd go for keeping this one small at this time.

[timreichen]

Seems like "0.0.0.0" is already the default in Deno.ListenOptions.

Suggested change

	using _listener = Deno.listen({
	port,
	hostname: options?.hostname ?? "0.0.0.0",
	});
	using _listener = Deno.listen({ port, hostname: options?.hostname });

[Xavrir]

Good catch. The literal hostname: options?.hostname trips exactOptionalPropertyTypes (passing hostname: undefined is not assignable to hostname?: string), so I forward the hostname only when it is set and otherwise omit it, letting Deno.ListenOptions apply its 0.0.0.0 default:

using _listener = Deno.listen(
  options?.hostname !== undefined
    ? { port, hostname: options.hostname }
    : { port },
);

That drops the redundant literal without widening what gets forwarded to Deno.listen. Updated in the latest push.