fix(serve): surface the real bind error instead of "Unexpected error / ServeError" - #38742

Issue for this PR

Closes #38738
Closes #38739

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Every bind failure in opencode serve printed the same two useless lines:

$ opencode serve --hostname 100.68.120.26 --port 4096
Error: Unexpected error

ServeError

No port, no hostname, no errno. A busy port, an address that isn't on the machine, and a privileged port were indistinguishable.

There are two separate reasons for that.

The errno is discarded. Effect's HttpServerError.ServeError is a Data.TaggedError, so its message is "" and the real failure sits on .cause. errorMessage() checked message then fell straight through to name — printing the literal string ServeError. Fixed by checking cause in between. That helps every tagged error, not just this one.

Bun reports the wrong errno anyway. Under Bun, node:http listen() returns EADDRINUSE for everything. Same script, two runtimes:

                        bun 1.3.14      node
100.68.120.26:45041     EADDRINUSE      EADDRNOTAVAIL
127.0.0.1:80            EADDRINUSE      EACCES
127.0.0.1:4096          EADDRINUSE      EADDRINUSE

Bun.serve() does the same, so it isn't the node:http shim specifically. This is why unwrapping the cause alone isn't enough — the errno you unwrap is wrong. So the new assertBindable() checks a literal-IP --hostname against os.networkInterfaces() before calling listen, where the answer is unambiguous. Hostnames are skipped, since listen resolves those.

bindError() handles what's left: it walks the cause chain and maps the errno. Sub-1024 ports are reported as a privilege problem rather than a conflict, because Bun folds EACCES into EADDRINUSE and a privileged port is the far likelier explanation down there.

Finally serve raises a CliError, so the message prints on its own instead of behind Unexpected error.

bind.ts throws plain Errors and has no CLI dependency on purpose — Server.listen is also called by the Desktop/WSL sidecar and by tests.

Worth noting the Bun errno behaviour is a Bun bug and should be reported there too. This PR doesn't depend on that being fixed.

How did you verify your code works?

Drove a real serve process through every case, on dev @ f516651:

Also reproduced the original bug on the released 1.17.9, not just on dev.

Tests:

• bun test test/server/ → 306 pass, 2 skip, 0 fail
• bun test test/cli/serve/ → 2 pass, 0 fail
• New test/server/bind.test.ts, 13 cases — including a fixture mimicking ServeError's empty-message shape, the sub-1024 case Bun mislabels, IPv6 zone ids, and a self-referential cause chain
• 3 new cases in packages/tui/test/util/error.test.ts for cause unwrapping, and that an explicit message still wins over cause
• tsc --noEmit clean, oxlint 0 warnings / 0 errors on all changed files

Screenshots / recordings

Not a UI change. CLI output is in the table above.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR