Fail unseal-key prompts on EOF

[sehkone]

Fixes the third of the tree's interactive readers. prompt_unseal_keys_interactive discarded the byte count read_line returns, so an exhausted stdin was indistinguishable from a blank line.

The reachable path is a plain operator mistake: answer the threshold, paste fewer shares than that, press Ctrl-D. Every remaining read returned Ok(0), the vector filled with empty strings, and save_unseal_keys truncated an existing unseal-keys.txt to blank lines, printed "saved" and exited zero. Nothing surfaced until read_unseal_keys_from_file filtered the blanks and bailed with Unseal key file is empty — by which point OpenBao is sealed and the shares are off the host.

Changes

• Check the count from read_line and bail with error_prompt_eof() at both prompts. A blank line the operator typed still behaves as it did.
• Read through an injected &mut dyn BufRead so both branches are testable without process stdin. prompt_unseal_keys_interactive keeps its name, parameters and pub(crate) visibility, so both call sites are untouched.
• A threshold that does not parse now carries error_invalid_unseal_threshold instead of error_prompt_read_failed, which was attached to a parse failure.
• Both prompt strings come from i18n. They were hardcoded in English while the declarations, accessors and Korean translations already existed, so BOOTROOT_LANG=ko rendered English here. The English strings are byte-identical, so default-locale output does not move.

Test plan

• Seven unit tests: EOF at the threshold, EOF right after the threshold (the reported destruction path), EOF partway through the shares, a full answer set, a supplied threshold skipping the threshold prompt, a non-numeric threshold, and a blank threshold staying a parse failure rather than becoming an EOF error.
• cargo test — all pass. The eight bootroot_rotate failures on my machine are the Docker daemon being absent (dial unix /var/run/docker.sock), unrelated to this change.
• cargo clippy --all-targets -- -D warnings and cargo fmt -- --check --config group_imports=StdExternalCrate are clean.
• Manual, both locales: BOOTROOT_LANG=ko bootroot openbao save-unseal-keys --secrets-dir <tempdir> renders the Korean prompts, and Ctrl-D exits nonzero without creating the file.

Preflight E2E was not run: this touches no Docker lifecycle, no E2E script and no code path they exercise — init prompts for unseal keys through its own reader, and the other caller of this one is guarded by is_terminal().

Closes #818