fix(proxy): stop blocking on orphaned stdio pipes after the child exits

[isacgalvao]

Summary

• The proxy path blocked forever after the wrapped command exited whenever a descendant of the child kept the inherited stdout/stderr pipe write ends open (build daemons, watchers, background jobs): the reader threads never saw EOF, so the unconditional join() never returned. Deterministic repro and full analysis in rtk proxy never returns after the child exits when a descendant keeps the stdout/stderr pipes open (Windows repro, same mechanism as #2320) #2829 (same mechanism as rtk dotnet build/test/restore hangs forever when a long-lived child daemon (NuGet auth plugin / MSBuild node) inherits the captured stdout/stderr pipe #2320, reproduced on Windows and unix).
• After child.wait() returns, the reader threads are now joined with a 2s grace period and detached if still blocked on an orphaned pipe; std::process::exit() in main() reaps them. Captured output moved to shared buffers so tracking still records exactly what was streamed when a reader is detached.
• Adds an integration test that leaves a quiet ~30s orphan holding the pipes and asserts rtk proxy returns promptly with the child's exit code (and, on unix, that the child's output was streamed through).

Fixes #2829

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test (Windows 11, stable)
• New test proxy_exits_when_child_dies_despite_orphaned_pipe fails on develop (hangs until the 15s deadline) and passes with this change
• Manual before/after: rtk proxy cmd /c "start /b waitfor ZZZ /t 20 & exit /b 0" — before: returns after ~20.2s (when the orphan dies; forever with a real daemon); after: returns in ~2s with exit code 0 and identical output
• Manual sanity: rtk proxy git status, rtk proxy cmd /c "echo hi & exit /b 3" — output and exit codes unchanged, no added latency on the normal EOF path

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull request overview

This PR fixes a long-standing hang in rtk proxy where the proxy process could block indefinitely after the wrapped command exited if any descendant process kept the inherited stdout/stderr pipe write-ends open (common with daemons/watchers/background jobs). The fix makes proxy completion driven by the direct child’s exit, with a bounded grace period for post-exit pipe draining, and adds a regression test for the orphaned-pipe scenario.

Changes:

• Update the proxy streaming implementation to join stdout/stderr reader threads with a 2s grace period after child.wait(), then detach if still blocked on orphaned pipes.
• Move captured stdout/stderr into shared buffers so tracking can still consume what was streamed before detaching.
• Add an integration test that reproduces the orphaned-pipe hang and asserts rtk proxy returns promptly with the child’s exit code (with a unix-only streamed-output assertion).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/main.rs	Implements bounded post-exit joining/detaching of stdout/stderr reader threads and shared capture buffers to prevent hangs on orphaned pipes.
tests/proxy_orphan_pipe_test.rs	Adds an integration regression test that leaves an orphan process holding pipes open and verifies rtk proxy exits promptly and forwards the exit code.