fix: retry attestation verification on transient GitHub API errors

[brynary]

Summary

• gh attestation verify during qlty upgrade occasionally fails due to transient HTTP 5xx from GitHub's attestation API
• Adds retry loop (up to 3 attempts) with linear backoff when stderr/stdout contains HTTP 5 or connection reset
• Non-transient failures (bad attestation, auth errors) still fail immediately

Test plan

• Verify cargo check passes
• Verify cargo test passes
• Manually test qlty upgrade --verify-attestations succeeds on a valid release

🤖 Generated with Claude Code

[qltysh]

Coverage Impact - ubuntu-latest

⬇️ Merging this pull request will decrease total coverage on main by 0.03%.

Modified Files with Diff Coverage (1)

Rating	File	% Diff	Uncovered Line #s
	qlty-cli/src/attestation.rs	0.0%	19-73
	Total	0.0%

🤖 Increase coverage with AI coding...

In the `verify-retry` branch, add test coverage for this new code:

- `qlty-cli/src/attestation.rs` -- Line 19-73

🚦 See full report on Qlty Cloud »

🛟 Help

• Diff Coverage: Coverage for added or modified lines of code (excludes deleted files). Learn more.

• Total Coverage: Coverage for the whole repository, calculated as the sum of all File Coverage. Learn more.

• File Coverage: Covered Lines divided by Covered Lines plus Missed Lines. (Excludes non-executable lines including blank lines and comments.)

  • Indirect Changes: Changes to File Coverage for files that were not modified in this PR. Learn more.

[qltysh]

Coverage Impact - macos-15

⬇️ Merging this pull request will decrease total coverage on main by 0.03%.

Modified Files with Diff Coverage (1)

Rating	File	% Diff	Uncovered Line #s
	qlty-cli/src/attestation.rs	0.0%	19-73
	Total	0.0%

🤖 Increase coverage with AI coding...

In the `verify-retry` branch, add test coverage for this new code:

- `qlty-cli/src/attestation.rs` -- Line 19-73

🚦 See full report on Qlty Cloud »

🛟 Help

• Diff Coverage: Coverage for added or modified lines of code (excludes deleted files). Learn more.

• Total Coverage: Coverage for the whole repository, calculated as the sum of all File Coverage. Learn more.

• File Coverage: Covered Lines divided by Covered Lines plus Missed Lines. (Excludes non-executable lines including blank lines and comments.)

  • Indirect Changes: Changes to File Coverage for files that were not modified in this PR. Learn more.