fix(gcp): protect minted key cleanup across interrupts

[cristim]

Summary

GCP setup now protects wizard-minted keys from SIGINT and SIGTERM from the final create choice through upload and cleanup. Prompts retain default interrupt behavior. Output occurs after cleanup, so stdout backpressure cannot strand the plaintext key.

This fixes the remaining interruption gap from #1947 and #2112. The other hardening items in #2117 remain separate.

Closes #2119.

Root cause and fix

The coordinator registered signal handling after Step 5 returned. Step 5 had already minted and written the key and could block while printing its temporary path. Another message could block after remote deletion but before local removal.

The coordinator now starts one signal context after the final input choice, before minting, and retains it through the existing uploader cleanup. A nil-safe deferred closure stops signal handling on failure. Successful cleanup restores default signal behavior before final output. Both blocking pre-cleanup messages are removed. Operator-supplied files retain their existing behavior.

Regression proof

Subprocess tests invoke the actual runConfigureGCP coordinator and real SDK clients against local fixture endpoints. They cover SIGINT and SIGTERM with stdout full, cancellation during the IAM request, and default SIGINT termination at the final prompt. Assertions check local file and directory removal and the remote delete request.

The tests fail against the original coordinator because plaintext survives both signals. Separate probes also fail when restoring only late signal activation, Step 5 output, or remote-cleanup output. The latter two leave the plaintext file present after the bounded subprocess timeout.

Verification

With Go 1.26.6 and GOWORK=off:

• Focused coordinator, upload, and key-writing tests with -race: pass.
• Full go test -race -short ./cmd/... -count=1: pass, 443.190s.
• Build, vet, and go mod tidy -diff: pass; tidy output empty.
• golangci-lint v2.10.1: zero issues.
• Normal commit hooks: pass.

These author checks cover tree f7b2822efae9f06add8ecae99229cb1fc075a5fe, identical at fully tested commit 2dc4d8e and follow-up candidate 42dcc9e5. Independent gpt-6-astra review approved final SHA 42dcc9e5e2cb6fc737ab7a1ec3791040825ddd09 with no actionable findings and independently passed the focused race tests in 8.234s. Tests use fixture credentials and local APIs; no live cloud operation was performed.

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📒 Files selected for processing (2)

• cmd/configure_gcp.go
• cmd/configure_gcp_signal_test.go

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