fix(network): surface launcher stderr on premature exit in background mode

[lwshang]

What

Follow-up to #675 for issue #597. In background mode the native launcher's stderr is redirected to a log file rather than inherited, so when it exits prematurely (e.g. its fixed gateway port is already in use by Docker) the user only saw:

network launcher at ... exited prematurely with status 101

with no indication of why.

This reads the launcher's captured stderr back and folds the tail into the LauncherExitedPrematurely error, so the cause travels with the error:

... exited prematurely with status 101
Launcher error output (from .../network-launcher/stderr.log):
<tail>

How

• New premature_exit_detail(background, stderr_file) in launcher.rs, called from the premature-exit arm of the select!:
  • Foreground: returns empty — stderr is already inherited and streamed live by fix: Stop suppressing launcher stdio #675, so the detail is shown, just rendered differently. No change.
  • Background: reads stderr.log, tails it (last 50 lines, then 8 KB, cut on a char boundary), and embeds it. Best-effort: on an unreadable/empty file it falls back to pointing at the log path rather than masking the exit status.
• Adds a detail: String field to the error variant and appends {detail} to the #[snafu(display)].

Tests

• Unit tests for the tail bounds (line cap, byte cap, trimming), the foreground no-op, the captured-output path, and the unreadable-file fallback.
• A #[cfg(unix)] integration test that occupies a port with a raw TcpListener (which passes icp-cli's own descriptor-based port check but makes the launcher fail to bind the gateway), runs network start --background, and asserts the captured output is attached to the error without pinning the exact launcher / pocket-ic wording. It also reaps any process spawned under the test's isolated temp home so it doesn't leak an orphaned pocket-ic.

Scope / follow-ups

Uses Refs #597 rather than Fixes because two related items are intentionally out of scope and are tracked separately:

1. Docker launcher path — spawn_docker_launcher has the same class of bug via a separate ContainerExitedPrematurely error and a different mechanism (needs docker logs, and can't be covered by this unix/native integration test). → network start: Docker launcher discards container output on premature exit (same as #597 for native) #677
2. Orphaned pocket-ic on the failure path — the launcher spawns pocket-ic and only cleans it up on the success path; on error/panic it's left running. → pocket-ic left orphaned when launcher exits on the error/panic path icp-cli-network-launcher#76

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Surfaces captured native launcher errors when background startup fails prematurely.

Changes:

• Appends a bounded stderr tail to premature-exit errors.
• Adds unit and integration coverage for error reporting.
• Cleans up orphaned test processes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
crates/icp/src/network/managed/launcher.rs	Reads, bounds, and reports launcher stderr.
crates/icp-cli/tests/network_tests.rs	Adds port-conflict regression coverage.

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