fix: secure large response spill files

[jbg]

Summary

• create large tool-response spill files with collision-safe random names
• keep spill files owner-only on Unix while preserving OS temporary-file cleanup semantics
• verify spill paths and contents in existing tests, plus add uniqueness and permission regressions

Security impact

Large tool responses can contain command output, file contents, credentials, or database results. Creating these files through tempfile prevents other local users from reading them under a permissive process umask and prevents pre-created-name or symlink collisions.

This intentionally keeps the files in OS temporary storage, following the lifecycle concern raised on #9308 rather than moving unbounded spill data into Goose's persistent data directory.

Testing

• cargo fmt --all -- --check
• cargo test -p goose agents::large_response_handler::tests -- --nocapture
• cargo build -p goose
• cargo clippy -p goose --all-targets -- -D warnings
• git diff --check

This finding was discovered by Project Loupe.

[jbg]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: f393ba0da5

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[michaelneale]

thanks, this reads well to me, nice one.