fix(auth): retry ExecProvider spawn on transient ETXTBSY

[jpage-godaddy]

Summary

• CI hit an intermittent panic in exec_provider_sends_request_to_stdin_and_parses_credential: Text file busy (os error 26) when exec'ing a freshly written, freshly chmod'd provider script.
• Root cause: a well-known Linux kernel race — ETXTBSY can be transiently reported when one thread execs a file while another thread in the same process forks at the same moment, even though nothing holds the file open for writing. Go's os/exec has retried on this since Go 1.9.
• ExecProvider::exec_raw had no retry around spawn(), so this is a real risk for any consumer that writes and execs a provider script under concurrent load, not just test flakiness.
• Fix: added ExecProvider::spawn_retrying_text_busy, which retries Command::spawn() up to 5 additional times (6 attempts total) with exponential backoff (1ms → 16ms between attempts) specifically on ErrorKind::ExecutableFileBusy, surfacing any other spawn error immediately.
• (Refined during review) Copilot flagged that the original loop spawned 5 times but only slept 4 times, leaving the computed 16ms backoff unused. Restructured the loop so all 5 retries actually sleep, maxing out at 16ms before the final attempt.

Failing run: https://github.com/godaddy/cli-engine/actions/runs/30959476281/job/92160057511?pr=83

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps
• cargo test --all-targets (all 282 tests in foundation.rs plus all other suites pass)
• cargo test --doc
• Re-ran the exec_provider_* suite multiple times to confirm stability
• Copilot review: 1 comment raised and fixed, re-reviewed clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Improves reliability of the ExecProvider auth provider by making process startup resilient to transient ETXTBSY (“Text file busy”) errors seen in CI and under concurrent exec load.

Changes:

• Wraps ExecProvider::exec_raw’s Command::spawn() call with a retry loop specifically for ErrorKind::ExecutableFileBusy.
• Adds an internal async helper (spawn_retrying_text_busy) implementing exponential backoff before retrying spawn.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.